light travel of

What is the speed of light?

The speed of light is the speed limit of the universe. Or is it?

What is a light-year?

• Speed of light FAQs
• Special relativity
• Faster than light
• Slowing down light
• Faster-than-light travel

Bibliography

The speed of light traveling through a vacuum is exactly 299,792,458 meters (983,571,056 feet) per second. That's about 186,282 miles per second — a universal constant known in equations as "c," or light speed. 

According to physicist Albert Einstein 's theory of special relativity , on which much of modern physics is based, nothing in the universe can travel faster than light. The theory states that as matter approaches the speed of light, the matter's mass becomes infinite. That means the speed of light functions as a speed limit on the whole universe . The speed of light is so immutable that, according to the U.S. National Institute of Standards and Technology , it is used to define international standard measurements like the meter (and by extension, the mile, the foot and the inch). Through some crafty equations, it also helps define the kilogram and the temperature unit Kelvin .

But despite the speed of light's reputation as a universal constant, scientists and science fiction writers alike spend time contemplating faster-than-light travel. So far no one's been able to demonstrate a real warp drive, but that hasn't slowed our collective hurtle toward new stories, new inventions and new realms of physics.

Related: Special relativity holds up to a high-energy test

A l ight-year is the distance that light can travel in one year — about 6 trillion miles (10 trillion kilometers). It's one way that astronomers and physicists measure immense distances across our universe.

Light travels from the moon to our eyes in about 1 second, which means the moon is about 1 light-second away. Sunlight takes about 8 minutes to reach our eyes, so the sun is about 8 light minutes away. Light from Alpha Centauri , which is the nearest star system to our own, requires roughly 4.3 years to get here, so Alpha Centauri is 4.3 light-years away.

"To obtain an idea of the size of a light-year, take the circumference of the Earth (24,900 miles), lay it out in a straight line, multiply the length of the line by 7.5 (the corresponding distance is one light-second), then place 31.6 million similar lines end to end," NASA's Glenn Research Center says on its website . "The resulting distance is almost 6 trillion (6,000,000,000,000) miles!"

Stars and other objects beyond our solar system lie anywhere from a few light-years to a few billion light-years away. And everything astronomers "see" in the distant universe is literally history. When astronomers study objects that are far away, they are seeing light that shows the objects as they existed at the time that light left them. 

This principle allows astronomers to see the universe as it looked after the Big Bang , which took place about 13.8 billion years ago. Objects that are 10 billion light-years away from us appear to astronomers as they looked 10 billion years ago — relatively soon after the beginning of the universe — rather than how they appear today.

Related: Why the universe is all history

Speed of light FAQs answered by an expert

We asked Rob Zellem, exoplanet-hunter and staff scientist at NASA's Jet Propulsion Lab, a few frequently asked questions about the speed of light. 

Dr. Rob Zellem is a staff scientist at NASA's Jet Propulsion Laboratory, a federally funded research and development center operated by the California Institute of Technology. Rob is the project lead for Exoplanet Watch, a citizen science project to observe exoplanets, planets outside of our own solar system, with small telescopes. He is also the Science Calibration lead for the Nancy Grace Roman Space Telescope's Coronagraph Instrument, which will directly image exoplanets. 

What is faster than the speed of light?

Nothing! Light is a "universal speed limit" and, according to Einstein's theory of relativity, is the fastest speed in the universe: 300,000 kilometers per second (186,000 miles per second). 

Is the speed of light constant?

The speed of light is a universal constant in a vacuum, like the vacuum of space. However, light *can* slow down slightly when it passes through an absorbing medium, like water (225,000 kilometers per second = 140,000 miles per second) or glass (200,000 kilometers per second = 124,000 miles per second). 

Who discovered the speed of light?

One of the first measurements of the speed of light was by Rømer in 1676 by observing the moons of Jupiter . The speed of light was first measured to high precision in 1879 by the Michelson-Morley Experiment. 

How do we know the speed of light?

Rømer was able to measure the speed of light by observing eclipses of Jupiter's moon Io. When Jupiter was closer to Earth, Rømer noted that eclipses of Io occurred slightly earlier than when Jupiter was farther away. Rømer attributed this effect due the time it takes for light to travel over the longer distance when Jupiter was farther from the Earth. 

How did we learn the speed of light?

As early as the 5th century, Greek philosophers like Empedocles and Aristotle disagreed on the nature of light speed. Empedocles proposed that light, whatever it was made of, must travel and therefore, must have a rate of travel. Aristotle wrote a rebuttal of Empedocles' view in his own treatise, On Sense and the Sensible , arguing that light, unlike sound and smell, must be instantaneous. Aristotle was wrong, of course, but it would take hundreds of years for anyone to prove it. 

In the mid 1600s, the Italian astronomer Galileo Galilei stood two people on hills less than a mile apart. Each person held a shielded lantern. One uncovered his lantern; when the other person saw the flash, he uncovered his too. But Galileo's experimental distance wasn't far enough for his participants to record the speed of light. He could only conclude that light traveled at least 10 times faster than sound.

In the 1670s, Danish astronomer Ole Rømer tried to create a reliable timetable for sailors at sea, and according to NASA , accidentally came up with a new best estimate for the speed of light. To create an astronomical clock, he recorded the precise timing of the eclipses of Jupiter's moon , Io, from Earth . Over time, Rømer observed that Io's eclipses often differed from his calculations. He noticed that the eclipses appeared to lag the most when Jupiter and Earth were moving away from one another, showed up ahead of time when the planets were approaching and occurred on schedule when the planets were at their closest or farthest points. This observation demonstrated what we today know as the Doppler effect, the change in frequency of light or sound emitted by a moving object that in the astronomical world manifests as the so-called redshift , the shift towards "redder", longer wavelengths in objects speeding away from us. In a leap of intuition, Rømer determined that light was taking measurable time to travel from Io to Earth. 

Rømer used his observations to estimate the speed of light. Since the size of the solar system and Earth's orbit wasn't yet accurately known, argued a 1998 paper in the American Journal of Physics , he was a bit off. But at last, scientists had a number to work with. Rømer's calculation put the speed of light at about 124,000 miles per second (200,000 km/s).

In 1728, English physicist James Bradley based a new set of calculations on the change in the apparent position of stars caused by Earth's travels around the sun. He estimated the speed of light at 185,000 miles per second (301,000 km/s) — accurate to within about 1% of the real value, according to the American Physical Society .

Two new attempts in the mid-1800s brought the problem back to Earth. French physicist Hippolyte Fizeau set a beam of light on a rapidly rotating toothed wheel, with a mirror set up 5 miles (8 km) away to reflect it back to its source. Varying the speed of the wheel allowed Fizeau to calculate how long it took for the light to travel out of the hole, to the adjacent mirror, and back through the gap. Another French physicist, Leon Foucault, used a rotating mirror rather than a wheel to perform essentially the same experiment. The two independent methods each came within about 1,000 miles per second (1,609 km/s) of the speed of light.

Another scientist who tackled the speed of light mystery was Poland-born Albert A. Michelson, who grew up in California during the state's gold rush period, and honed his interest in physics while attending the U.S. Naval Academy, according to the University of Virginia . In 1879, he attempted to replicate Foucault's method of determining the speed of light, but Michelson increased the distance between mirrors and used extremely high-quality mirrors and lenses. Michelson's result of 186,355 miles per second (299,910 km/s) was accepted as the most accurate measurement of the speed of light for 40 years, until Michelson re-measured it himself. In his second round of experiments, Michelson flashed lights between two mountain tops with carefully measured distances to get a more precise estimate. And in his third attempt just before his death in 1931, according to the Smithsonian's Air and Space magazine, he built a mile-long depressurized tube of corrugated steel pipe. The pipe simulated a near-vacuum that would remove any effect of air on light speed for an even finer measurement, which in the end was just slightly lower than the accepted value of the speed of light today. 

Michelson also studied the nature of light itself, wrote astrophysicist Ethan Siegal in the Forbes science blog, Starts With a Bang . The best minds in physics at the time of Michelson's experiments were divided: Was light a wave or a particle? 

Michelson, along with his colleague Edward Morley, worked under the assumption that light moved as a wave, just like sound. And just as sound needs particles to move, Michelson and Morley and other physicists of the time reasoned, light must have some kind of medium to move through. This invisible, undetectable stuff was called the "luminiferous aether" (also known as "ether"). 

Though Michelson and Morley built a sophisticated interferometer (a very basic version of the instrument used today in LIGO facilities), Michelson could not find evidence of any kind of luminiferous aether whatsoever. Light, he determined, can and does travel through a vacuum.

"The experiment — and Michelson's body of work — was so revolutionary that he became the only person in history to have won a Nobel Prize for a very precise non-discovery of anything," Siegal wrote. "The experiment itself may have been a complete failure, but what we learned from it was a greater boon to humanity and our understanding of the universe than any success would have been!"

Special relativity and the speed of light

Einstein's theory of special relativity unified energy, matter and the speed of light in a famous equation: E = mc^2. The equation describes the relationship between mass and energy — small amounts of mass (m) contain, or are made up of, an inherently enormous amount of energy (E). (That's what makes nuclear bombs so powerful: They're converting mass into blasts of energy.) Because energy is equal to mass times the speed of light squared, the speed of light serves as a conversion factor, explaining exactly how much energy must be within matter. And because the speed of light is such a huge number, even small amounts of mass must equate to vast quantities of energy.

In order to accurately describe the universe, Einstein's elegant equation requires the speed of light to be an immutable constant. Einstein asserted that light moved through a vacuum, not any kind of luminiferous aether, and in such a way that it moved at the same speed no matter the speed of the observer. 

Think of it like this: Observers sitting on a train could look at a train moving along a parallel track and think of its relative movement to themselves as zero. But observers moving nearly the speed of light would still perceive light as moving away from them at more than 670 million mph. (That's because moving really, really fast is one of the only confirmed methods of time travel — time actually slows down for those observers, who will age slower and perceive fewer moments than an observer moving slowly.)

In other words, Einstein proposed that the speed of light doesn't vary with the time or place that you measure it, or how fast you yourself are moving. 

Therefore, objects with mass cannot ever reach the speed of light. If an object ever did reach the speed of light, its mass would become infinite. And as a result, the energy required to move the object would also become infinite: an impossibility.

That means if we base our understanding of physics on special relativity (which most modern physicists do), the speed of light is the immutable speed limit of our universe — the fastest that anything can travel. 

What goes faster than the speed of light?

Although the speed of light is often referred to as the universe's speed limit, the universe actually expands even faster. The universe expands at a little more than 42 miles (68 kilometers) per second for each megaparsec of distance from the observer, wrote astrophysicist Paul Sutter in a previous article for Space.com . (A megaparsec is 3.26 million light-years — a really long way.) 

In other words, a galaxy 1 megaparsec away appears to be traveling away from the Milky Way at a speed of 42 miles per second (68 km/s), while a galaxy two megaparsecs away recedes at nearly 86 miles per second (136 km/s), and so on. 

"At some point, at some obscene distance, the speed tips over the scales and exceeds the speed of light, all from the natural, regular expansion of space," Sutter explained. "It seems like it should be illegal, doesn't it?"

Special relativity provides an absolute speed limit within the universe, according to Sutter, but Einstein's 1915 theory regarding general relativity allows different behavior when the physics you're examining are no longer "local."

"A galaxy on the far side of the universe? That's the domain of general relativity, and general relativity says: Who cares! That galaxy can have any speed it wants, as long as it stays way far away, and not up next to your face," Sutter wrote. "Special relativity doesn't care about the speed — superluminal or otherwise — of a distant galaxy. And neither should you."

Does light ever slow down?

Light in a vacuum is generally held to travel at an absolute speed, but light traveling through any material can be slowed down. The amount that a material slows down light is called its refractive index. Light bends when coming into contact with particles, which results in a decrease in speed.

For example, light traveling through Earth's atmosphere moves almost as fast as light in a vacuum, slowing down by just three ten-thousandths of the speed of light. But light passing through a diamond slows to less than half its typical speed, PBS NOVA reported. Even so, it travels through the gem at over 277 million mph (almost 124,000 km/s) — enough to make a difference, but still incredibly fast.

Light can be trapped — and even stopped — inside ultra-cold clouds of atoms, according to a 2001 study published in the journal Nature . More recently, a 2018 study published in the journal Physical Review Letters proposed a new way to stop light in its tracks at "exceptional points," or places where two separate light emissions intersect and merge into one.

Researchers have also tried to slow down light even when it's traveling through a vacuum. A team of Scottish scientists successfully slowed down a single photon, or particle of light, even as it moved through a vacuum, as described in their 2015 study published in the journal Science . In their measurements, the difference between the slowed photon and a "regular" photon was just a few millionths of a meter, but it demonstrated that light in a vacuum can be slower than the official speed of light. 

Can we travel faster than light?

— Spaceship could fly faster than light

— Here's what the speed of light looks like in slow motion

— Why is the speed of light the way it is?

Science fiction loves the idea of "warp speed." Faster-than-light travel makes countless sci-fi franchises possible, condensing the vast expanses of space and letting characters pop back and forth between star systems with ease. 

But while faster-than-light travel isn't guaranteed impossible, we'd need to harness some pretty exotic physics to make it work. Luckily for sci-fi enthusiasts and theoretical physicists alike, there are lots of avenues to explore.

All we have to do is figure out how to not move ourselves — since special relativity would ensure we'd be long destroyed before we reached high enough speed — but instead, move the space around us. Easy, right? 

One proposed idea involves a spaceship that could fold a space-time bubble around itself. Sounds great, both in theory and in fiction.

"If Captain Kirk were constrained to move at the speed of our fastest rockets, it would take him a hundred thousand years just to get to the next star system," said Seth Shostak, an astronomer at the Search for Extraterrestrial Intelligence (SETI) Institute in Mountain View, California, in a 2010 interview with Space.com's sister site LiveScience . "So science fiction has long postulated a way to beat the speed of light barrier so the story can move a little more quickly."

Without faster-than-light travel, any "Star Trek" (or "Star War," for that matter) would be impossible. If humanity is ever to reach the farthest — and constantly expanding — corners of our universe, it will be up to future physicists to boldly go where no one has gone before.

Additional resources

For more on the speed of light, check out this fun tool from Academo that lets you visualize how fast light can travel from any place on Earth to any other. If you’re more interested in other important numbers, get familiar with the universal constants that define standard systems of measurement around the world with the National Institute of Standards and Technology . And if you’d like more on the history of the speed of light, check out the book " Lightspeed: The Ghostly Aether and the Race to Measure the Speed of Light " (Oxford, 2019) by John C. H. Spence.

Aristotle. “On Sense and the Sensible.” The Internet Classics Archive, 350AD. http://classics.mit.edu/Aristotle/sense.2.2.html .

D’Alto, Nick. “The Pipeline That Measured the Speed of Light.” Smithsonian Magazine, January 2017. https://www.smithsonianmag.com/air-space-magazine/18_fm2017-oo-180961669/ .

Fowler, Michael. “Speed of Light.” Modern Physics. University of Virginia. Accessed January 13, 2022. https://galileo.phys.virginia.edu/classes/252/spedlite.html#Albert%20Abraham%20Michelson .

Giovannini, Daniel, Jacquiline Romero, Václav Potoček, Gergely Ferenczi, Fiona Speirits, Stephen M. Barnett, Daniele Faccio, and Miles J. Padgett. “Spatially Structured Photons That Travel in Free Space Slower than the Speed of Light.” Science, February 20, 2015. https://www.science.org/doi/abs/10.1126/science.aaa3035 .

Goldzak, Tamar, Alexei A. Mailybaev, and Nimrod Moiseyev. “Light Stops at Exceptional Points.” Physical Review Letters 120, no. 1 (January 3, 2018): 013901. https://doi.org/10.1103/PhysRevLett.120.013901 . 

Hazen, Robert. “What Makes Diamond Sparkle?” PBS NOVA, January 31, 2000. https://www.pbs.org/wgbh/nova/article/diamond-science/ . 

“How Long Is a Light-Year?” Glenn Learning Technologies Project, May 13, 2021. https://www.grc.nasa.gov/www/k-12/Numbers/Math/Mathematical_Thinking/how_long_is_a_light_year.htm . 

American Physical Society News. “July 1849: Fizeau Publishes Results of Speed of Light Experiment,” July 2010. http://www.aps.org/publications/apsnews/201007/physicshistory.cfm . 

Liu, Chien, Zachary Dutton, Cyrus H. Behroozi, and Lene Vestergaard Hau. “Observation of Coherent Optical Information Storage in an Atomic Medium Using Halted Light Pulses.” Nature 409, no. 6819 (January 2001): 490–93. https://doi.org/10.1038/35054017 . 

NIST. “Meet the Constants.” October 12, 2018. https://www.nist.gov/si-redefinition/meet-constants . 

Ouellette, Jennifer. “A Brief History of the Speed of Light.” PBS NOVA, February 27, 2015. https://www.pbs.org/wgbh/nova/article/brief-history-speed-light/ . 

Shea, James H. “Ole Ro/Mer, the Speed of Light, the Apparent Period of Io, the Doppler Effect, and the Dynamics of Earth and Jupiter.” American Journal of Physics 66, no. 7 (July 1, 1998): 561–69. https://doi.org/10.1119/1.19020 . 

Siegel, Ethan. “The Failed Experiment That Changed The World.” Forbes, April 21, 2017. https://www.forbes.com/sites/startswithabang/2017/04/21/the-failed-experiment-that-changed-the-world/ . 

Stern, David. “Rømer and the Speed of Light,” October 17, 2016. https://pwg.gsfc.nasa.gov/stargaze/Sun4Adop1.htm . 

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Vicky Stein is a science writer based in California. She has a bachelor's degree in ecology and evolutionary biology from Dartmouth College and a graduate certificate in science writing from the University of California, Santa Cruz (2018). Afterwards, she worked as a news assistant for PBS NewsHour, and now works as a freelancer covering anything from asteroids to zebras. Follow her most recent work (and most recent pictures of nudibranchs) on Twitter. 

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What is the speed of light? Here’s the history, discovery of the cosmic speed limit

On one hand, the speed of light is just a number: 299,792,458 meters per second. And on the other, it’s one of the most important constants that appears in nature and defines the relationship of causality itself.

As far as we can measure, it is a constant. It is the same speed for every observer in the entire universe. This constancy was first established in the late 1800’s with the experiments of Albert Michelson and Edward Morley at Case Western Reserve University . They attempted to measure changes in the speed of light as the Earth orbited around the Sun. They found no such variation, and no experiment ever since then has either.

Observations of the cosmic microwave background, the light released when the universe was 380,000 years old, show that the speed of light hasn’t measurably changed in over 13.8 billion years.

In fact, we now define the speed of light to be a constant, with a precise speed of 299,792,458 meters per second. While it remains a remote possibility in deeply theoretical physics that light may not be a constant, for all known purposes it is a constant, so it’s better to just define it and move on with life.

How was the speed of light first measured?

In 1676 the Danish astronomer Ole Christensen Romer made the first quantitative measurement of how fast light travels. He carefully observed the orbit of Io, the innermost moon of Jupiter. As the Earth circles the Sun in its own orbit, sometimes it approaches Jupiter and sometimes it recedes away from it. When the Earth is approaching Jupiter, the path that light has to travel from Io is shorter than when the Earth is receding away from Jupiter. By carefully measuring the changes to Io’s orbital period, Romer calculated a speed of light of around 220,000 kilometers per second.

Observations continued to improve until by the 19 th century astronomers and physicists had developed the sophistication to get very close to the modern value. In 1865, James Clerk Maxwell made a remarkable discovery. He was investigating the properties of electricity and magnetism, which for decades had remained mysterious in unconnected laboratory experiments around the world. Maxwell found that electricity and magnetism were really two sides of the same coin, both manifestations of a single electromagnetic force.

As Maxwell explored the consequences of his new theory, he found that changing magnetic fields can lead to changing electric fields, which then lead to a new round of changing magnetic fields. The fields leapfrog over each other and can even travel through empty space. When Maxwell went to calculate the speed of these electromagnetic waves, he was surprised to see the speed of light pop out – the first theoretical calculation of this important number.

What is the most precise measurement of the speed of light?

Because it is defined to be a constant, there’s no need to measure it further. The number we’ve defined is it, with no uncertainty, no error bars. It’s done. But the speed of light is just that – a speed. The number we choose to represent it depends on the units we use: kilometers versus miles, seconds versus hours, and so on. In fact, physicists commonly just set the speed of light to be 1 to make their calculations easier. So instead of trying to measure the speed light travels, physicists turn to more precisely measuring other units, like the length of the meter or the duration of the second. In other words, the defined value of the speed of light is used to establish the length of other units like the meter.

How does light slow down?

Yes, the speed of light is always a constant. But it slows down whenever it travels through a medium like air or water. How does this work? There are a few different ways to present an answer to this question, depending on whether you prefer a particle-like picture or a wave-like picture.

In a particle-like picture, light is made of tiny little bullets called photons. All those photons always travel at the speed of light, but as light passes through a medium those photons get all tangled up, bouncing around among all the molecules of the medium. This slows down the overall propagation of light, because it takes more time for the group of photons to make it through.

In a wave-like picture, light is made of electromagnetic waves. When these waves pass through a medium, they get all the charged particles in motion, which in turn generate new electromagnetic waves of their own. These interfere with the original light, forcing it to slow down as it passes through.

Either way, light always travels at the same speed, but matter can interfere with its travel, making it slow down.

Why is the speed of light important?

The speed of light is important because it’s about way more than, well, the speed of light. In the early 1900’s Einstein realized just how special this speed is. The old physics, dominated by the work of Isaac Newton, said that the universe had a fixed reference frame from which we could measure all motion. This is why Michelson and Morley went looking for changes in the speed, because it should change depending on our point of view. But their experiments showed that the speed was always constant, so what gives?

Einstein decided to take this experiment at face value. He assumed that the speed of light is a true, fundamental constant. No matter where you are, no matter how fast you’re moving, you’ll always see the same speed.

This is wild to think about. If you’re traveling at 99% the speed of light and turn on a flashlight, the beam will race ahead of you at…exactly the speed of light, no more, no less. If you’re coming from the opposite direction, you’ll still also measure the exact same speed.

This constancy forms the basis of Einstein’s special theory of relativity, which tells us that while all motion is relative – different observers won’t always agree on the length of measurements or the duration of events – some things are truly universal, like the speed of light.

Can you go faster than light speed?

Nope. Nothing can. Any particle with zero mass must travel at light speed. But anything with mass (which is most of the universe) cannot. The problem is relativity. The faster you go, the more energy you have. But we know from Einstein’s relativity that energy and mass are the same thing. So the more energy you have, the more mass you have, which makes it harder for you to go even faster. You can get as close as you want to the speed of light, but to actually crack that barrier takes an infinite amount of energy. So don’t even try.

How is the speed at which light travels related to causality?

If you think you can find a cheat to get around the limitations of light speed, then I need to tell you about its role in special relativity. You see, it’s not just about light. It just so happens that light travels at this special speed, and it was the first thing we discovered to travel at this speed. So it could have had another name. Indeed, a better name for this speed might be “the speed of time.”

Related: Is time travel possible? An astrophysicist explains

We live in a universe of causes and effects. All effects are preceded by a cause, and all causes lead to effects. The speed of light limits how quickly causes can lead to effects. Because it’s a maximum speed limit for any motion or interaction, in a given amount of time there’s a limit to what I can influence. If I want to tap you on the shoulder and you’re right next to me, I can do it right away. But if you’re on the other side of the planet, I have to travel there first. The motion of me traveling to you is limited by the speed of light, so that sets how quickly I can tap you on the shoulder – the speed light travels dictates how quickly a single cause can create an effect.

The ability to go faster than light would allow effects to happen before their causes. In essence, time travel into the past would be possible with faster-than-light travel. Since we view time as the unbroken chain of causes and effects going from the past to the future, breaking the speed of light would break causality, which would seriously undermine our sense of the forward motion of time.

Why does light travel at this speed?

No clue. It appears to us as a fundamental constant of nature. We have no theory of physics that explains its existence or why it has the value that it does. We hope that a future understanding of nature will provide this explanation, but right now all investigations are purely theoretical. For now, we just have to take it as a given.

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May 20, 2016

How does light travel?

by Matt Williams, Universe Today

Ever since Democritus – a Greek philosopher who lived between the 5th and 4th century's BCE – argued that all of existence was made up of tiny indivisible atoms, scientists have been speculating as to the true nature of light. Whereas scientists ventured back and forth between the notion that light was a particle or a wave until the modern, the 20th century led to breakthroughs that showed that it behaves as both.

These included the discovery of the electron, the development of quantum theory, and Einstein's Theory of Relativity. However, there remains many fascinating and unanswered questions when it comes to light, many of which arise from its dual nature. For instance, how is it that light can be apparently without mass, but still behave as a particle? And how can it behave like a wave and pass through a vacuum, when all other waves require a medium to propagate?

Theory of Light in the 19th Century:

During the Scientific Revolution, scientists began moving away from Aristotelian scientific theories that had been seen as accepted canon for centuries. This included rejecting Aristotle's theory of light, which viewed it as being a disturbance in the air (one of his four "elements" that composed matter), and embracing the more mechanistic view that light was composed of indivisible atoms.

In many ways, this theory had been previewed by atomists of Classical Antiquity – such as Democritus and Lucretius – both of whom viewed light as a unit of matter given off by the sun. By the 17th century, several scientists emerged who accepted this view, stating that light was made up of discrete particles (or "corpuscles"). This included Pierre Gassendi, a contemporary of René Descartes, Thomas Hobbes, Robert Boyle, and most famously, Sir Isaac Newton.

Newton's corpuscular theory was an elaboration of his view of reality as an interaction of material points through forces. This theory would remain the accepted scientific view for more than 100 years, the principles of which were explained in his 1704 treatise "Opticks, or, a Treatise of the Reflections, Refractions, Inflections, and Colours of Light". According to Newton, the principles of light could be summed as follows:

• Every source of light emits large numbers of tiny particles known as corpuscles in a medium surrounding the source.
• These corpuscles are perfectly elastic, rigid, and weightless.

This represented a challenge to "wave theory", which had been advocated by 17th century Dutch astronomer Christiaan Huygens. . These theories were first communicated in 1678 to the Paris Academy of Sciences and were published in 1690 in his "Traité de la lumière" ("Treatise on Light"). In it, he argued a revised version of Descartes views, in which the speed of light is infinite and propagated by means of spherical waves emitted along the wave front.

Double-Slit Experiment:

By the early 19th century, scientists began to break with corpuscular theory. This was due in part to the fact that corpuscular theory failed to adequately explain the diffraction, interference and polarization of light, but was also because of various experiments that seemed to confirm the still-competing view that light behaved as a wave.

The most famous of these was arguably the Double-Slit Experiment, which was originally conducted by English polymath Thomas Young in 1801 (though Sir Isaac Newton is believed to have conducted something similar in his own time). In Young's version of the experiment, he used a slip of paper with slits cut into it, and then pointed a light source at them to measure how light passed through it.

According to classical (i.e. Newtonian) particle theory, the results of the experiment should have corresponded to the slits, the impacts on the screen appearing in two vertical lines. Instead, the results showed that the coherent beams of light were interfering, creating a pattern of bright and dark bands on the screen. This contradicted classical particle theory, in which particles do not interfere with each other, but merely collide.

The only possible explanation for this pattern of interference was that the light beams were in fact behaving as waves. Thus, this experiment dispelled the notion that light consisted of corpuscles and played a vital part in the acceptance of the wave theory of light. However subsequent research, involving the discovery of the electron and electromagnetic radiation , would lead to scientists considering yet again that light behaved as a particle too, thus giving rise to wave-particle duality theory.

Electromagnetism and Special Relativity:

Prior to the 19th and 20th centuries, the speed of light had already been determined. The first recorded measurements were performed by Danish astronomer Ole Rømer, who demonstrated in 1676 using light measurements from Jupiter's moon Io to show that light travels at a finite speed (rather than instantaneously).

By the late 19th century , James Clerk Maxwell proposed that light was an electromagnetic wave, and devised several equations (known as Maxwell's equations) to describe how electric and magnetic fields are generated and altered by each other and by charges and currents. By conducting measurements of different types of radiation (magnetic fields, ultraviolet and infrared radiation), he was able to calculate the speed of light in a vacuum (represented as c).

In 1905, Albert Einstein published "On the Electrodynamics of Moving Bodies", in which he advanced one of his most famous theories and overturned centuries of accepted notions and orthodoxies. In his paper, he postulated that the speed of light was the same in all inertial reference frames, regardless of the motion of the light source or the position of the observer.

Exploring the consequences of this theory is what led him to propose his theory of Special Relativity, which reconciled Maxwell's equations for electricity and magnetism with the laws of mechanics, simplified the mathematical calculations, and accorded with the directly observed speed of light and accounted for the observed aberrations. It also demonstrated that the speed of light had relevance outside the context of light and electromagnetism.

For one, it introduced the idea that major changes occur when things move close the speed of light, including the time-space frame of a moving body appearing to slow down and contract in the direction of motion when measured in the frame of the observer. After centuries of increasingly precise measurements, the speed of light was determined to be 299,792,458 m/s in 1975.

Einstein and the Photon:

In 1905, Einstein also helped to resolve a great deal of confusion surrounding the behavior of electromagnetic radiation when he proposed that electrons are emitted from atoms when they absorb energy from light. Known as the photoelectric effect, Einstein based his idea on Planck's earlier work with "black bodies" – materials that absorb electromagnetic energy instead of reflecting it (i.e. white bodies).

At the time, Einstein's photoelectric effect was attempt to explain the "black body problem", in which a black body emits electromagnetic radiation due to the object's heat. This was a persistent problem in the world of physics, arising from the discovery of the electron, which had only happened eight years previous (thanks to British physicists led by J.J. Thompson and experiments using cathode ray tubes).

At the time, scientists still believed that electromagnetic energy behaved as a wave, and were therefore hoping to be able to explain it in terms of classical physics. Einstein's explanation represented a break with this, asserting that electromagnetic radiation behaved in ways that were consistent with a particle – a quantized form of light which he named "photons". For this discovery, Einstein was awarded the Nobel Prize in 1921.

Wave-Particle Duality:

Subsequent theories on the behavior of light would further refine this idea, which included French physicist Louis-Victor de Broglie calculating the wavelength at which light functioned. This was followed by Heisenberg's "uncertainty principle" (which stated that measuring the position of a photon accurately would disturb measurements of it momentum and vice versa), and Schrödinger's paradox that claimed that all particles have a " wave function ".

In accordance with quantum mechanical explanation, Schrodinger proposed that all the information about a particle (in this case, a photon) is encoded in its wave function, a complex-valued function roughly analogous to the amplitude of a wave at each point in space. At some location, the measurement of the wave function will randomly "collapse", or rather "decohere", to a sharply peaked function. This was illustrated in Schrödinger famous paradox involving a closed box, a cat, and a vial of poison (known as the "Schrödinger's Cat" paradox).

According to his theory, wave function also evolves according to a differential equation (aka. the Schrödinger equation). For particles with mass, this equation has solutions; but for particles with no mass, no solution existed. Further experiments involving the Double-Slit Experiment confirmed the dual nature of photons. where measuring devices were incorporated to observe the photons as they passed through the slits.

When this was done, the photons appeared in the form of particles and their impacts on the screen corresponded to the slits – tiny particle-sized spots distributed in straight vertical lines. By placing an observation device in place, the wave function of the photons collapsed and the light behaved as classical particles once more. As predicted by Schrödinger, this could only be resolved by claiming that light has a wave function, and that observing it causes the range of behavioral possibilities to collapse to the point where its behavior becomes predictable.

The development of Quantum Field Theory (QFT) was devised in the following decades to resolve much of the ambiguity around wave-particle duality. And in time, this theory was shown to apply to other particles and fundamental forces of interaction (such as weak and strong nuclear forces). Today, photons are part of the Standard Model of particle physics, where they are classified as boson – a class of subatomic particles that are force carriers and have no mass.

So how does light travel? Basically, traveling at incredible speeds (299 792 458 m/s) and at different wavelengths, depending on its energy. It also behaves as both a wave and a particle, able to propagate through mediums (like air and water) as well as space. It has no mass, but can still be absorbed, reflected, or refracted if it comes in contact with a medium. And in the end, the only thing that can truly slow down or arrest the speed of light is gravity (i.e. a black hole).

What we have learned about light and electromagnetism has been intrinsic to the revolution which took place in physics in the early 20th century, a revolution that we have been grappling with ever since. Thanks to the efforts of scientists like Maxwell, Planck, Einstein, Heisenberg and Schrodinger, we have learned much, but still have much to learn.

For instance, its interaction with gravity (along with weak and strong nuclear forces) remains a mystery. Unlocking this, and thus discovering a Theory of Everything (ToE) is something astronomers and physicists look forward to. Someday, we just might have it all figured out!

Source: Universe Today

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What is the speed of light?

Light is faster than anything else in the known universe, though its speed can change depending on what it's passing through.

The universe has a speed limit, and it's the speed of light. Nothing can travel faster than light — not even our best spacecraft — according to the laws of physics.

So, what is the speed of light? 

Light moves at an incredible 186,000 miles per second (300,000 kilometers per second), equivalent to almost 700 million mph (more than 1 billion km/h). That's fast enough to circumnavigate the globe 7.5 times in one second, while a typical passenger jet would take more than two days to go around once (and that doesn't include stops for fuel or layovers!). 

Light moves so fast that, for much of human history, we thought it traveled instantaneously. As early as the late 1600s, though, scientist Ole Roemer was able to measure the speed of light (usually referred to as c ) by using observations of Jupiter's moons, according to Britannica . 

Around the turn of the 19th century, physicist James Clerk Maxwell created his theories of electromagnetism . Light is itself made up of electric and magnetic fields, so electromagnetism could describe the behavior and motion of light — including its theoretical speed. That value was 299,788 kilometers per second, with a margin of error of plus or minus 30. In the 1970s, physicists used lasers to measure the speed of light with much greater precision, leaving an error of only 0.001. Nowadays, the speed of light is used to define units of length, so its value is fixed; humans have essentially agreed the speed of light is 299,792.458 kilometers per second, exactly.

Light doesn't always have to go so fast, though. Depending on what it's traveling through — air, water, diamonds, etc. — it can slow down. The official speed of light is measured as if it's traveling in a vacuum, a space with no air or anything to get in the way. You can most clearly see differences in the speed of light in something like a prism, where certain energies of light bend more than others, creating a rainbow.

— How many moons does Earth have ?

— What would happen if the moon were twice as close to Earth?

— If you're on the moon, does the Earth appear to go through phases?

Interestingly, the speed of light is no match for the vast distances of space, which is itself a vacuum. It takes 8 minutes for light from the sun to reach Earth, and a couple years for light from the other closest stars (like Proxima Centauri) to get to our planet. This is why astronomers use the unit light-years — the distance light can travel in one year — to measure vast distances in space.

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Because of this universal speed limit, telescopes are essentially time machines . When astronomers look at a star 500 light-years away, they're looking at light from 500 years ago. Light from around 13 billion light-years away (equivalently, 13 billion years ago) shows up as the cosmic microwave background, remnant radiation from the Big Bang in the universe's infancy. The speed of light isn't just a quirk of physics; it has enabled modern astronomy as we know it, and it shapes the way we see the world — literally.

Briley Lewis (she/her) is a freelance science writer and Ph.D. Candidate/NSF Fellow at the University of California, Los Angeles studying Astronomy & Astrophysics. Follow her on Twitter  @briles_34 or visit her website  www.briley-lewis.com .

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• Kooperkieri54 That's correct. In a vacuum, such as outer space, light travels at a constant speed of approximately 299,792 kilometers per second (or about 186,282 miles per second), which is often rounded to 300,000 kilometers per second for simplicity. This speed is commonly referred to as the speed of light in a vacuum and is denoted by the symbol "c". However, when light passes through a medium, such as air, water, or glass, its speed can change. This change in speed is due to the interaction of light with the atoms or molecules in the medium. The speed of light in a medium is typically slower than its speed in a vacuum because the particles in the medium can absorb and re-emit photons, causing a delay in the overall propagation of light. The change in speed of light in different materials is characterized by the refractive index of the material. The refractive index indicates how much the speed of light is reduced when it passes through that particular material compared to its speed in a vacuum. It's worth noting that while light is the fastest known phenomenon in the universe, it is not instantaneous . what pickleball paddles do the prose use. It still takes time for light to travel from one point to another, and its speed is an essential aspect of many fundamental theories and principles in physics. Reply
• marcuso I thought the speed of an event was relative, with all observers having their own space time, therfore how does this fit into 2 observers seeing the same speed of light ? Reply
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Light moves through the universe at the fastest speed astronomers can measure. In fact, the speed of light is a cosmic speed limit, and nothing is known to move faster. How fast does light move? This limit can be measured and it also helps define our understanding of the universe's size and age.

What Is Light: Wave or Particle?

Light travels fast, at a velocity of 299, 792, 458 meters per second. How can it do this? To understand that, it's helpful to know what light actually is and that's largely a 20th-century discovery.

The nature of light was a great mystery for centuries. Scientists had trouble grasping the concept of its wave and particle nature. If it was a wave what did it propagate through? Why did it appear to travel at the same speed in all directions? And, what can the speed of light tell us about the cosmos? It wasn't until Albert Einstein described this theory of special relativity in 1905 it all came into focus. Einstein argued that space and time were relative and that the speed of light was the constant that connected the two.

What Is the Speed of Light?

It is often stated that the speed of light is constant and that nothing can travel faster than the speed of light. This isn't entirely accurate. The value of 299,792,458 meters per second (186,282 miles per second) is the speed of light in a vacuum. However, light actually slows down as it passes through different media. For instance, when it moves through glass, it slows down to about two-thirds of its speed in a vacuum. Even in air, which is nearly a vacuum, light slows down slightly. As it moves through space, it encounters clouds of gas and dust, as well as gravitational fields, and those can change the speed a tiny bit. The clouds of gas and dust also absorb some of the light as it passes through.

This phenomenon has to do with the nature of light, which is an electromagnetic wave. As it propagates through a material its electric and magnetic fields "disturb" the charged particles that it comes in contact with. These disturbances then cause the particles to radiate light at the same frequency, but with a phase shift. The sum of all these waves produced by the "disturbances" will lead to an electromagnetic wave with the same frequency as the original light, but with a shorter wavelength and, hence a slower speed.

Interesting, as fast as light moves, its path can be bent as it passes by regions in space with intense gravitational fields. This is fairly easily seen in galaxy clusters, which contain a lot of matter (including dark matter), which warps the path of light from more distant objects, such as quasars.

Lightspeed and Gravitational Waves

Current theories of physics predict that gravitational waves also travel at the speed of light, but this is still being confirmed as scientists study the phenomenon of gravitational waves from colliding black holes and neutron stars. Otherwise, there are no other objects that travel that fast. Theoretically, they can get close to the speed of light, but not faster.

One exception to this may be space-time itself. It appears that distant galaxies are moving away from us faster than the speed of light. This is a "problem" that scientists are still trying to understand. However, one interesting consequence of this is that a travel system based on the idea of a warp drive . In such a technology, a spacecraft is at rest relative to space and it's actually space that moves, like a surfer riding a wave on the ocean. Theoretically, this might allow for superluminal travel. Of course, there are other practical and technological limitations that stand in the way, but it's an interesting science-fiction idea that is getting some scientific interest. 

Travel Times for Light

One of the questions that astronomers get from members of the public is: "how long would it take light to go from object X to Object Y?" Light gives them a very accurate way to measure the size of the universe by defining distances. Here are a few of the common ones distance measurements:

• The Earth to the Moon : 1.255 seconds
• The Sun to Earth : 8.3 minutes
• Our Sun to the next closest star : 4.24 years
• Across our Milky Way  galaxy : 100,000 years
• To the closest  spiral galaxy (Andromeda) : 2.5 million years
• Limit of the observable universe to Earth : 13.8 billion years

Interestingly, there are objects that are beyond our ability to see simply because the universe IS expanding, and some are "over the horizon" beyond which we cannot see. They will never come into our view, no matter how fast their light travels. This is one of the fascinating effects of living in an expanding universe. 

Edited by Carolyn Collins Petersen

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• Fundamental Physical Constants
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• Can Humans Hear Sound in Space?
• Time Travel: Dream or Possible Reality?
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• How to Define Acceleration
• Wave-Particle Duality - Definition
• How Redshift Shows the Universe is Expanding

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• Sound & Light (Physics): How are They Different?

How Does Light Travel?

Sound & Light (Physics): How are They Different?

The question of how light travels through space is one of the perennial mysteries of physics. In modern explanations, it is a wave phenomenon that doesn't need a medium through which to propagate. According to quantum theory, it also behaves as a collection of particles under certain circumstances. For most macroscopic purposes, though, its behavior can be described by treating it as a wave and applying the principles of wave mechanics to describe its motion.

Electromagnetic Vibrations

In the mid 1800s, Scottish physicist James Clerk Maxwell established that light is a form of electromagnetic energy that travels in waves. The question of how it manages to do so in the absence of a medium is explained by the nature of electromagnetic vibrations. When a charged particle vibrates, it produces an electrical vibration that automatically induces a magnetic one -- physicists often visualize these vibrations occurring in perpendicular planes. The paired oscillations propagate outward from the source; no medium, except for the electromagnetic field that permeates the universe, is required to conduct them.

A Ray of Light

When an electromagnetic source generates light, the light travels outward as a series of concentric spheres spaced in accordance with the vibration of the source. Light always takes the shortest path between a source and destination. A line drawn from the source to the destination, perpendicular to the wave-fronts, is called a ray. Far from the source, spherical wave fronts degenerate into a series of parallel lines moving in the direction of the ray. Their spacing defines the wavelength of the light, and the number of such lines that pass a given point in a given unit of time defines the frequency.

The Speed of Light

The frequency with which a light source vibrates determines the frequency -- and wavelength -- of the resultant radiation. This directly affects the energy of the wave packet -- or burst of waves moving as a unit -- according to a relationship established by physicist Max Planck in the early 1900s. If the light is visible, the frequency of vibration determines color. The speed of light is unaffected by vibrational frequency, however. In a vacuum, it is always 299,792 kilometers per second (186, 282 miles per second), a value denoted by the letter "c." According to Einstein's Theory of Relativity, nothing in the universe travels faster than this.

Refraction and Rainbows

Light travels slower in a medium than it does in a vacuum, and the speed is proportional to the density of the medium. This speed variation causes light to bend at the interface of two media -- a phenomenon called refraction. The angle at which it bends depends on the densities of the two media and the wavelength of the incident light. When light incident on a transparent medium is composed of wave fronts of different wavelengths, each wave front bends at a different angle, and the result is a rainbow.

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About the Author

Chris Deziel holds a Bachelor's degree in physics and a Master's degree in Humanities, He has taught science, math and English at the university level, both in his native Canada and in Japan. He began writing online in 2010, offering information in scientific, cultural and practical topics. His writing covers science, math and home improvement and design, as well as religion and the oriental healing arts.

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Wave-Particle Duality: An Overview

Physical optics vs. geometric optics: definition & differences.

15.1 The Electromagnetic Spectrum

Section learning objectives.

By the end of this section, you will be able to do the following:

• Define the electromagnetic spectrum, and describe it in terms of frequencies and wavelengths
• Describe and explain the differences and similarities of each section of the electromagnetic spectrum and the applications of radiation from those sections

Teacher Support

The learning objectives in this section will help your students master the following standards

• (A) examine and describe oscillatory motion and wave propagation in various types of media;
• (B) investigate and analyze characteristics of waves, including velocity, frequency, amplitude, and wavelength, and calculate using the relationship between wave speed, frequency, and wavelength;
• (C) compare characteristics and behaviors of transverse waves, including electromagnetic waves and the electromagnetic spectrum, and characteristics and behaviors of longitudinal waves, including sound waves; and
• (F) describe the role of wave characteristics and behaviors in medical and industrial applications.

In addition, the High School Physics Laboratory Manual addresses content in this section in the lab titled: Light and Color, as well as the following standards:

• (C) compare characteristics and behaviors of transverse waves, including electromagnetic waves and the electromagnetic spectrum, and characteristics and behaviors of longitudinal waves, including sound waves.
• (B) compare and explain the emission spectra produced by various atoms.

Section Key Terms

[BL] Explain that the term spectrum refers to a physical property that has a broad range with values that are continuous in some cases and, in other cases, discrete. Ask for other examples of spectra, for example, sound, people’s heights, etc.

[OL] Ask students to name ways that sunlight affects Earth. Provide examples that students don’t name: photosynthesis, weather, climate, seasons, warming, etc. Discuss energy transformations that take place after light enters the atmosphere, such as transformations in food chains and ecosystems. Ask students if they can explain how the energy in fossil fuels was originally light energy.

Misconception Alert

The light we can see is called visible light. Dispel any misconceptions that visible light is somehow different from radiation we cannot see, except for frequency and wavelength. The fact that some radiation is visible has to do with how the eye functions, not with the radiation itself.

The Electromagnetic Spectrum

We generally take light for granted, but it is a truly amazing and mysterious form of energy. Think about it: Light travels to Earth across millions of kilometers of empty space. When it reaches us, it interacts with matter in various ways to generate almost all the energy needed to support life, provide heat, and cause weather patterns. Light is a form of electromagnetic radiation (EMR) . The term light usually refers to visible light , but this is not the only form of EMR. As we will see, visible light occupies a narrow band in a broad range of types of electromagnetic radiation.

[OL] Discuss electric, magnetic, and gravitational fields. Point out how these three fields are similar, and how they differ.

[AL] Describe vectors as having magnitude and direction, and explain that fields are vector quantities. In these cases, the fields are made up of forces acting in a direction.

Electromagnetic radiation is generated by a moving electric charge, that is, by an electric current. As you will see when you study electricity, an electric current generates both an electric field , E , and a magnetic field , B . These fields are perpendicular to each other. When the moving charge oscillates, as in an alternating current, an EM wave is propagated. Figure 15.2 shows how an electromagnetic wave moves away from the source—indicated by the ~ symbol.

[BL] Review wave properties: frequency, wavelength, and amplitude. Ask students to recall sound and water waves, and explain how they relate to these properties.

[OL] Explain that an important difference between EM waves and other waves is that they can travel across empty space.

[AL] Ask if students remember the differences between longitudinal and transverse waves. Give examples. Explain that waves carry energy, not matter.

Watch Physics

Electromagnetic waves and the electromagnetic spectrum.

This video, link below, is closely related to the following figure. If you have questions about EM wave properties, the EM spectrum, how waves propagate, or definitions of any of the related terms, the answers can be found in this video .

Grasp Check

In an electromagnetic wave, how are the magnetic field, the electric field, and the direction of propagation oriented to each other?

• All three are parallel to each other and are along the x -axis.
• All three are mutually perpendicular to each other.
• The electric field and magnetic fields are parallel to each other and perpendicular to the direction of propagation.
• The magnetic field and direction of propagation are parallel to each other along the y -axis and perpendicular to the electric field.

Direct students to use this video as a way of connecting to the information in the following two figures, as well as to the following table.

Virtual Physics

Radio waves and electromagnetic fields.

This simulation demonstrates wave propagation. The EM wave is propagated from the broadcast tower on the left, just as in Figure 15.2 . You can make the wave yourself or allow the animation to send it. When the wave reaches the antenna on the right, it causes an oscillating current. This is how radio and television signals are transmitted and received.

Where do radio waves fall on the electromagnetic spectrum?

• Radio waves have the same wavelengths as visible light.
• Radio waves fall on the high-frequency side of visible light.
• Radio waves fall on the short-wavelength side of visible light.
• Radio waves fall on the low-frequency side of visible light.

Connect the discussion from the previous video, in which the generation of an electromagnetic wave is described, to this application of transmission and reception of electromagnetic waves. In particular, point out how the reception of the radio wave is essentially identical to the method by which the wave is generated. Explain also that these electromagnetic waves are the carrier waves on which audio or visual signals—either analog or digital—are placed, so that they can be transmitted to receivers within a certain range of the broadcast antenna.

From your study of sound waves, recall these features that apply to all types of waves:

• Wavelength —The distance between two wave crests or two wave troughs, expressed in various metric measures of distance
• Frequency —The number of wave crests that pass a point per second, expressed in hertz (Hz or s –1 )
• Amplitude : The height of the crest above the null point

As mentioned, electromagnetic radiation takes several forms. These forms are characterized by a range of frequencies. Because frequency is inversely proportional to wavelength, any form of EMR can also be represented by its range of wavelengths. Figure 15.3 shows the frequency and wavelength ranges of various types of EMR. With how many of these types are you familiar?

Take a few minutes to study the positions of the various types of radiation on the EM spectrum, above. The narrow band that is visible light extends from lower-frequency red light to higher-frequency violet light. Frequencies just below the visible are called infrared (below red) and those just above are ultraviolet (beyond violet). Radio waves , which overlap with the frequencies used for media broadcasts of TV and radio signals, occupy frequencies even lower than infrared (IR). The microwave radiation that you see on the diagram is the same radiation that is used in a microwave oven. What we feel as radiant heat is also a form of low-frequency EMR. The high-frequency radiation to the right of ultraviolet (UV) includes X-rays and gamma (γ) rays.

[BL] Notice that most harmful forms of EM radiation are on the high-frequency end of the spectrum.

[OL] Ask which forms of EM radiation students have heard about. Ask them to describe the types of radiation they remember, and correct any misconceptions. Discuss the difference between ionizing radiation and nonionizing radiation, and the difference between electromagnetic radiation and other types of radiation—alpha, beta, etc.

Heat waves, a type of infrared radiation, are basically no different from other EM waves. We feel them as heat because they have a frequency that interacts with our bodies in a way that transforms EM energy into thermal energy.

Boundless Physics

Maxwell’s Equations

The Scottish physicist James Clerk Maxwell (1831–1879) is regarded widely to have been the greatest theoretical physicist of the nineteenth century. Although he died young, Maxwell not only formulated a complete electromagnetic theory, represented by Maxwell’s equations , he also developed the kinetic theory of gases, and made significant contributions to the understanding of color vision and the nature of Saturn’s rings.

Maxwell brought together all the work that had been done by brilliant physicists, such as Ørsted, Coulomb, Ampere, Gauss, and Faraday, and added his own insights to develop the overarching theory of electromagnetism. Maxwell’s equations are paraphrased here in words because their mathematical content is beyond the level of this text. However, the equations illustrate how apparently simple mathematical statements can elegantly unite and express a multitude of concepts—why mathematics is the language of science.

• Electric field lines originate on positive charges and terminate on negative charges. The electric field is defined as the force per unit charge on a test charge, and the strength of the force is related to the electric constant, ε 0 .
• Magnetic field lines are continuous, having no beginning or end. No magnetic monopoles are known to exist. The strength of the magnetic force is related to the magnetic constant, μ 0 .
• A changing magnetic field induces an electromotive force (emf) and, hence, an electric field. The direction of the emf opposes the change, changing direction of the magnetic field.
• Magnetic fields are generated by moving charges or by changing electric fields.

Maxwell’s complete theory shows that electric and magnetic forces are not separate, but different manifestations of the same thing—the electromagnetic force. This classical unification of forces is one motivation for current attempts to unify the four basic forces in nature—the gravitational, electromagnetic, strong nuclear, and weak nuclear forces. The weak nuclear and electromagnetic forces have been unified, and further unification with the strong nuclear force is expected; but, the unification of the gravitational force with the other three has proven to be a real head-scratcher.

One final accomplishment of Maxwell was his development in 1855 of a process that could produce color photographic images. In 1861, he and photographer Thomas Sutton worked together on this process. The color image was achieved by projecting red, blue, and green light through black-and-white photographs of a tartan ribbon, each photo itself exposed in different-colored light. The final image was projected onto a screen (see Figure 15.4 ).

Features that encouraged mathematicians and physicists to accept Maxwell’s equations is that they are seen as being both elegant and—considering the difference between an electric charge and a magnetic dipole, which give rise to the respective fields—essentially symmetrical. When scientists are looking for an approach to developing a new theory, they usually begin with the simplest and most symmetrical explanations. An example of such symmetry is the fact that electrons and protons have equal and opposite charges. You can see the symmetry in the four statements, given above, that describe the equations. The first two statements show a similar treatment of electric and magnetic fields, and the last two describe how a magnetic field can generate an electric field, and vice versa.

From our present-day perspective, we can now see the significance of Maxwell’s equations. This was the first step in the quest to unify all natural forces under one theory. After Maxwell unified the electric and magnetic forces as the electromagnetic force, others unified this force with the weak nuclear force, and there is evidence that the strong nuclear force can be unified with the electroweak force. The only force that has resisted unification with the others is the gravitational force. A theory that would unify all forces is often referred as a grand unified theory or a theory of everything . The quest for such a theory is still underway.

• According to Maxwell’s equations, electromagnetic force gives rise to electric force and magnetic force.
• According to Maxwell’s equations, electric force and magnetic force are different manifestations of electromagnetic force.
• According to Maxwell’s equations, electric force is the cause of electromagnetic force.
• According to Maxwell’s equations, magnetic force is the cause of electromagnetic force.

Characteristics of Electromagnetic Radiation

All the EM waves mentioned above are basically the same form of radiation. They can all travel across empty space, and they all travel at the speed of light in a vacuum. The basic difference between types of radiation is their differing frequencies. Each frequency has an associated wavelength. As frequency increases across the spectrum, wavelength decreases. Energy also increases with frequency. Because of this, higher frequencies penetrate matter more readily. Some of the properties and uses of the various EM spectrum bands are listed in Table 15.1 .

[BL] Explain transparency and opacity. Discuss how some materials are transparent to certain frequencies but opaque to others. Ask students for examples of materials that can be penetrated by some EM frequencies but not by others. Ask for examples of materials that are transparent to visible light and materials that are opaque to visible light.

[OL] Ask students why a lead apron is laid across dental patients during dental X-rays. Explain that X-rays are at the high-energy end of the spectrum and that they are very penetrating. They are only stopped by very dense materials, such as lead.

[AL] Ask if students can explain Earth’s greenhouse effect in terms of the penetrating power of various frequencies of EM radiation. Explain that the atmosphere is more transparent to visible light than to heat waves. Visible light penetrates the atmosphere and warms Earth’s surface. The heated surface radiates heat waves, which are trapped partially by certain gases in the atmosphere.

The narrow band of visible light is a combination of the colors of the rainbow. Figure 15.5 shows the section of the EM spectrum that includes visible light. The frequencies corresponding to these wavelengths are 4.0 × 10 14  s −1 4.0 × 10 14  s −1 at the red end to 7.9 × 10 14  s −1 7.9 × 10 14  s −1 at the violet end. This is a very narrow range, considering that the EM spectrum spans about 20 orders of magnitude.

[BL] Review the primary and secondary colors of pigments. Note that this is subtractive color mixing.

[OL] Explain the difference between subtractive and additive color mixing. The colors on the subtractive color wheel are made by pigments that absorb all colors but one. Therefore, when these colors all overlap, all light is absorbed and the result is black. White light is a combination of all colors, so when all colors are added together on the additive color wheel, the result is white. Explain that cyan is a shade of blue and that magenta is a shade of red.

Tips For Success

Wavelengths of visible light are often given in nanometers, nm. One nm equals 10 −9 10 −9 m. For example, yellow light has a wavelength of about 600 nm, or 6 × 10 −7 6 × 10 −7 m.

As a child, you probably learned the color wheel, shown on the left in Figure 15.6 . It helps if you know what color results when you mix different colors of paint together. Mixing two of the primary pigment colors—magenta, yellow, or cyan—together results in a secondary color. For example, mixing cyan and yellow makes green. This is called subtractive color mixing. Mixing different colors of light together is quite different. The diagram on the right shows additive color mixing. In this case, the primary colors are red, green, and blue, and the secondary colors are cyan, magenta, and yellow. Mixing pigments and mixing light are different because materials absorb light by a different set of rules than does the perception of light by the eye. Notice that, when all colors are subtracted, the result is no color, or black. When all colors are added, the result is white light. We see the reverse of this when white sunlight is separated into the visible spectrum by a prism or by raindrops when a rainbow appears in the sky.

Color Vision

This video demonstrates additive color and color filters. Try all the settings except Photons .

• A blue filter absorbs blue light, causing the observed light to be a combination of the other colors.
• A blue filter absorbs the opposite color of light—orange, causing the observed light to be blue.
• A blue filter permits only blue light to pass though, absorbing the other colors and leaving blue light for the observer.
• A blue filter permits only the opposite color light—orange—to pass through, leaving orange light for the observer.

Have students adjust the different colored lights for the RGB bulb simulation, first with individual settings, then with combinations of two and three colors to see what colors result and are perceived. Similarly, with the Single Bulb simulation, have students note how different filter settings affect what colors are seen for light with different color components.

Links To Physics

Animal color perception.

The physics of color perception has interesting links to zoology. Other animals have very different views of the world than humans, especially with respect to which colors can be seen. Color is detected by cells in the eye called cones . Humans have three cones that are sensitive to three different ranges of electromagnetic wavelengths. They are called red, blue, and green cones, although these colors do not correspond exactly to the centers of the three ranges. The ranges of wavelengths that each cone detects are red, 500 to 700 nm; green, 450 to 630 nm; and blue, 400 to 500 nm.

Most primates also have three kinds of cones and see the world much as we do. Most mammals other than primates only have two cones and have a less colorful view of things. Dogs, for example see blue and yellow, but are color blind to red and green. You might think that simpler species, such as fish and insects, would have less sophisticated vision, but this is not the case. Many birds, reptiles, amphibians, and insects have four or five different cones in their eyes. These species don’t have a wider range of perceived colors, but they see more hues, or combinations of colors. Also, some animals, such as bees or rattlesnakes, see a range of colors that is as broad as ours, but shifted into the ultraviolet or infrared.

These differences in color perception are generally adaptations that help the animals survive. Colorful tropical birds and fish display some colors that are too subtle for us to see. These colors are believed to play a role in the species mating rituals. Figure 15.7 shows the colors visible and the color range of vision in humans, bees, and dogs.

The symbiotic relationship between plants and their pollinators—bees, birds, etc.—is related to color perception. Plants have evolved to have flowers with colors that bees can see easily, and bees can find those flowers easily to collect the nectar they need for survival.

The belief that bulls are enraged by seeing the color red is a misconception. What did you read in this Links to Physics that shows why this belief is incorrect?

• Bulls are color-blind to every color in the spectrum of colors.
• Bulls are color-blind to the blue colors in the spectrum of colors.
• Bulls are color-blind to the red colors in the spectrum of colors.
• Bulls are color-blind to the green colors in the spectrum of colors.

Humans have found uses for every part of the electromagnetic spectrum. We will take a look at the uses of each range of frequencies, beginning with visible light. Most of our uses of visible light are obvious; without it our interaction with our surroundings would be much different. We might forget that nearly all of our food depends on the photosynthesis process in plants, and that the energy for this process comes from the visible part of the spectrum. Without photosynthesis, we would also have almost no oxygen in the atmosphere.

[BL] Ask how different frequencies of EM radiation are applied. Name each frequency range, and ask the students to supply the application, for example, X-rays used in medical imaging.

[OL] Ask students if they know why low-frequency radiation generally has different uses than high-frequency radiation. Explain that it has to do with penetrating power, which is related to health hazards.

[AL] Mention the ranges of TV signals designated very high frequency (VHF) and ultrahigh frequency (UHF). Explain that these frequencies are only relatively high compared to radio broadcast frequencies. Their place in the whole EM spectrum is at the low end.

The low-frequency, infrared region of the spectrum has many applications in media broadcasting. Television, radio, cell phone, and remote-control devices all broadcast and/or receive signals with these wavelengths. AM and FM radio signals are both low-frequency radiation. They are in different regions of the spectrum, but that is not their basic difference. AM and FM are abbreviations for amplitude modulation and frequency modulation . Information in AM signals has the form of changes in amplitude of the radio waves; information in FM signals has the form of changes in wave frequency .

Another application of long-wavelength radiation is found in microwave ovens. These appliances cook or warm food by irradiating it with EM radiation in the microwave frequency range. Most kitchen microwaves use a frequency of 2.45 × 10 9 2.45 × 10 9 Hz. These waves have the right amount of energy to cause polar molecules, such as water, to rotate faster. Polar molecules are those that have a partial charge separation. The rotational energy of these molecules is given up to surrounding matter as heat. The first microwave ovens were called Radaranges because they were based on radar technology developed during World War II.

Radar uses radiation with wavelengths similar to those of microwaves to detect the location and speed of distant objects, such as airplanes, weather formations, and motor vehicles. Radar information is obtained by receiving and analyzing the echoes of microwaves reflected by an object. The speed of the object can be measured using the Doppler shift of the returning waves. This is the same effect you learned about when you studied sound waves. Like sound waves, EM waves are shifted to higher frequencies by an object moving toward an observer, and to lower frequencies by an object moving away from the observer. Astronomers use this same Doppler effect to measure the speed at which distant galaxies are moving away from us. In this case, the shift in frequency is called the red shift , because visible frequencies are shifted toward the lower-frequency, red end of the spectrum.

Exposure to any radiation with frequencies greater than those of visible light carries some health hazards. All types of radiation in this range are known to cause cell damage. The danger is related to the high energy and penetrating ability of these EM waves. The likelihood of being harmed by any of this radiation depends largely on the amount of exposure. Most people try to reduce exposure to UV radiation from sunlight by using sunscreen and protective clothing. Physicians still use X-rays to diagnose medical problems, but the intensity of the radiation used is extremely low. Figure 15.8 shows an X-ray image of a patient’s chest cavity.

One medical-imaging technique that involves no danger of exposure is magnetic resonance imaging (MRI). MRI is an important imaging and research tool in medicine, producing highly detailed two- and three-dimensional images. Radio waves are broadcast, absorbed, and reemitted in a resonance process that is sensitive to the density of nuclei, usually hydrogen nuclei—protons.

Check Your Understanding

Use these questions to assess student achievement of the section’s Learning Objectives. If students are struggling with a specific objective, these questions will help identify any gaps and direct students to the relevant content.

Identify the fields produced by a moving charged particle.

• Both an electric field and a magnetic field will be produced.
• Neither a magnetic field nor an electric field will be produced.
• A magnetic field, but no electric field will be produced.
• Only the electric field, but no magnetic field will be produced.
• Visible light has higher frequencies and shorter wavelengths than X-rays.
• Visible light has lower frequencies and shorter wavelengths than X-rays.
• Visible light has higher frequencies and longer wavelengths than X-rays.
• Visible light has lower frequencies and longer wavelengths than X-rays.
• The wavelength increases.
• The wavelength first increases and then decreases.
• The wavelength first decreases and then increases.
• The wavelength decreases.
• X-rays have higher penetrating energy than radio waves.
• X-rays have lower penetrating energy than radio waves.
• X-rays have a lower frequency range than radio waves.
• X-rays have longer wavelengths than radio waves.
• both an electric field and a magnetic field
• neither a magnetic field nor an electric field
• only a magnetic field, but no electric field
• only an electric field, but no magnetic field

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Want to cite, share, or modify this book? This book uses the Creative Commons Attribution License and you must attribute Texas Education Agency (TEA). The original material is available at: https://www.texasgateway.org/book/tea-physics . Changes were made to the original material, including updates to art, structure, and other content updates.

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• Authors: Paul Peter Urone, Roger Hinrichs
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• Book title: Physics
• Publication date: Mar 26, 2020
• Location: Houston, Texas
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How Does Light Travel? Does It Travel Forever?

Last Updated on Jan 27 2023

Light is almost always around us, but it’s not something that most people think twice about. 

If you do start thinking about it, you might wonder how light travels and for how far. The answer is that light moves as a wave, and it can go on forever. Let’s go over what that means and dive a little deeper into the topic.

• How Does Light Travel?

Since light travels like a wave, it can travel through a vacuum without interacting with anything. However, when light does go through something, that object can absorb some of it. Light travels through these objects, like glass and water, leaving heat behind.

Think of a flashlight . When you turn it on and face it toward a pool, the light can travel through the pool. However, if the flashlight isn’t bright enough, it won’t illuminate the bottom of the pool.

It’s also worth noting that light travels in all directions. If you have a lightbulb in the middle of the room with no lampshade, the light is going to travel in every direction. This is a basic principle of light, and it’s why when we create light to use, we must find ways to direct it.

Both flashlights and lamps do this, and it’s how we control where the light goes, by reflecting it off the surface of the object.

• How Far Does Light Travel?

Light can travel for infinity. It doesn’t have a set range, and that’s why we can see light from billions of miles away.

It’s also why when we point the James Webb or the Hubble telescope out into deep space, we can see even farther, observing the light from galaxies from the very early universe.

However, when light hits objects like planets or other matter, this either reflects or absorbs the light. The most extreme example of this is a black hole. If light hits a black hole , the gravity is so strong that it can’t escape, thus ending the distance that the light travels.

• Other Things That Affect the Movement of Light

Sometimes, gravity can pull on light without completely absorbing it or reflecting it.

This is common with black holes, neutron stars, and even large stars. As light passes by these objects, their respective gravities can pull on the light. If the light is far enough away, it will continue to move away from the object, but it will have a new trajectory.

If you look at a ray of light from above, what it will look like when this is happening is that the light will bend. So, while light travels in a straight line in all directions from an object, the gravitational pull of different objects can shift where that light ends up.

• Final Thoughts

When you’re trying to figure out the science in the world, it can all seem a bit complicated. Hopefully, now that you know more about how light moves, you can move on to new questions and contemplate the ways that different things work.

That’s what science is all about, and if you’re asking questions like how light moves, an interest in science might just be in your future!

Featured Image Credit: nepool, Shutterstock

Table of Contents

About the Author Robert Sparks

Robert’s obsession with all things optical started early in life, when his optician father would bring home prototypes for Robert to play with. Nowadays, Robert is dedicated to helping others find the right optics for their needs. His hobbies include astronomy, astrophysics, and model building. Originally from Newark, NJ, he resides in Santa Fe, New Mexico, where the nighttime skies are filled with glittering stars.

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The Nature of Light

Introduction.

Light is a transverse, electromagnetic wave that can be seen by the typical human. The wave nature of light was first illustrated through experiments on diffraction and interference . Like all electromagnetic waves, light can travel through a vacuum. The transverse nature of light can be demonstrated through polarization .

• In 1678, Christiaan Huygens (1629–1695) published Traité de la Lumiere , where he argued in favor of the wave nature of light. Huygens stated that an expanding sphere of light behaves as if each point on the wave front were a new source of radiation of the same frequency and phase.
• Thomas Young (1773–1829) and Augustin-Jean Fresnel (1788–1827) disproved Newton's corpuscular theory.

Light is produced by one of two methods…

• Incandescence is the emission of light from "hot" matter (T ≳ 800 K).
• Luminescence is the emission of light when excited electrons fall to lower energy levels (in matter that may or may not be "hot").

Just notes so far. The speed of light in a vacuum is represented by the letter c from the Latin celeritas — swiftness. Measurements of the speed of light.

Veramente non l'ho sperimentata, salvo che in lontananza piccola, cioè manco d'un miglio, dal che non ho potuto assicurarmi se veramente la comparsa del lume opposto sia instantanea; ma ben, se non instantanea, velocissima….   In fact I have tried the experiment only at a short distance, less than a mile, from which I have not been able to ascertain with certainty whether the appearance of the opposite light was instantaneous or not; but if not instantaneous it is extraordinarily rapid ….       Galileo Galilei, 1638 Galileo Galilei, 1638

Ole Rømer (1644–1710) Denmark. "Démonstration touchant le mouvement de la lumière trouvé par M. Roemer de l'Académie des Sciences." Journal des Scavans . 7 December 1676. Rømer's idea was to use the transits of Jupiter's moon Io to determine the time. Not local time, which was already possible, but a "universal" time that would be the same for all observers on the Earth, Knowing the standard time would allow one to determine one's longitude on the Earth — a handy thing to know when navigating the featureless oceans.

Unfortunately, Io did not turn out to be a good clock. Rømer observed that times between eclipses got shorter as Earth approached Jupiter, and longer as Earth moved farther away. He hypothesized that this variation was due to the time it took for light to travel the lesser or greater distance, and estimated that the time for light to travel the diameter of the Earth's orbit, a distance of two astronomical units, was 22 minutes.

• The speed of light in a vacuum is a universal constant in all reference frames.
• The speed of light in a vacuum is fixed at 299,792,458 m/s by the current definition of the meter.
• The speed of light in a medium is always slower the speed of light in a vacuum.
• The speed of light depends upon the medium through which it travels.The speed of anything with mass is always less than the speed of light in a vacuum.

other characteristics

The amplitude of a light wave is related to its intensity.

• Intensity is the absolute measure of a light wave's power density.
• Brightness is the relative intensity as perceived by the average human eye.

The frequency of a light wave is related to its color.

• Color is such a complex topic that it has its own section in this book.
• Laser light is effectively monochromatic.
• There are six simple, named colors in English (and many other languages) each associated with a band of monochromatic light. In order of increasing frequency they are red, orange, yellow, green, blue, and violet .
• Light is sometimes also known as visible light to contrast it from "ultraviolet light" and "infrared light"
• Other forms of electromagnetic radiation that are not visible to humans are sometimes also known informally as "light"
• Nearly every light source is polychromatic.
• White light is polychromatic.

A graph of relative intensity vs. frequency is called a spectrum (plural: spectra ). Although frequently associated with light, the term can be applied to any wave phenomena.

• Blackbody radiators emit a continuous spectrum.
• The excited electrons in a gas emit a discrete spectrum.

The wavelength of a light wave is inversely proportional to its frequency.

• Light is often described by it's wavelength in a vacuum .
• Light ranges in wavelength from 400 nm on the violet end to 700 nm on the red end of the visible spectrum.

Phase differences between light waves can produce visible interference effects. (There are several sections in this book on interference phenomena and light.)

Leftovers about animals.

• Falcon can see a 10 cm. object from a distance of 1.5 km.
• Fly's Eye has a flicker fusion rate of 300/s. Humans have a flicker fusion rate of only 60/s in bright light and 24/s in dim light. The flicker fusion rate is the frequency with which the "flicker" of an image cannot be distinguished as an individual event. Like the frame of a movie… if you slowed it down, you would see individual frames. Speed it up and you see a constantly moving image. Octopus' eye has a flicker fusion frequency of 70/s in bright light.
• Penguin has a flat cornea that allows for clear vision underwater. Penguins can also see into the ultraviolet range of the electromagnetic spectrum.
• Sparrow Retina has 400,000 photoreceptors per square. mm.
• Reindeer can see ultraviolet wavelengths, which may help them view contrasts in their mostly white environment.

How Light Travels: The Reason Why Telescopes Can See the Invisible Parts of Our Universe

Due to how light travels, we can only see the most eye-popping details of space—like nebulas, supernovas, and black holes—with specialized telescopes.

• Our eyes can see only a tiny fraction of these wavelengths , but our instruments enable us to learn far more.
• Here, we outline how various telescopes detect different wavelengths of light from space.

Light travels only one way: in a straight line. But the path it takes from Point A to Point B is always a waveform, with higher-energy light traveling in shorter wavelengths. Photons , which are tiny parcels of energy, have been traveling across the universe since they first exploded from the Big Bang . They always travel through the vacuum of space at 186,400 miles per second—the speed of light—which is faster than anything else.

Too bad we can glimpse only about 0.0035 percent of the light in the universe with our naked eyes. Humans can perceive just a tiny sliver of the electromagnetic spectrum: wavelengths from about 380–750 nanometers. This is what we call the visible part of the electromagnetic spectrum. The universe may be lovely to look at in this band, but our vision skips right over vast ranges of wavelengths that are either shorter or longer than this limited range. On either side of the visible band lies evidence of interstellar gas clouds, the hottest stars in the universe, gas clouds between galaxies , the gas that rushes into black holes, and much more.

Fortunately, telescopes allow us to see what would otherwise remain hidden. To perceive gas clouds between stars and galaxies, we use detectors that can capture infrared wavelengths. Super-hot stars require instruments that see short, ultraviolet wavelengths. To see the gas clouds between galaxies, we need X-ray detectors.

We’ve been using telescopes designed to reveal the invisible parts of the cosmos for more than 60 years. Because Earth’s atmosphere absorbs most wavelengths of light, many of our telescopes must observe the cosmos from orbit or outer space.

Here’s a snapshot of how we use specialized detectors to explore how light travels across the universe.

Infrared Waves

We can’t see infrared waves, but we can feel them as heat . A sensitive detector like the James Webb Space Telescope can discern this thermal energy from far across the universe. But we use infrared in more down-to-Earth ways as well. For example, remote-control devices work by sending infrared signals at about 940 nanometers to your television or stereo. These heat waves also emanate from incubators to help hatch a chick or keep a pet reptile warm. As a warm being, you radiate infrared waves too; a person using night vision goggles can see you, because the goggles turn infrared energy into false-color optical energy that your eyes can perceive. Infrared telescopes let us see outer space in a similar way.

Astronomers began the first sky surveys with infrared telescopes in the 1960s and 1970s. Webb , launched in 2021, takes advantage of the infrared spectrum to probe the deepest regions of the universe. Orbiting the sun at a truly cold expanse—about one million miles from Earth—Webb has three infrared detectors with the ability to peer farther back in time than any other telescope has so far.

Its primary imaging device, the Near Infrared Camera (NIRCam), observes the universe through detectors tuned to incoming wavelengths ranging from 0.6 to 5 microns, ideal for seeing light from the universe’s earliest stars and galaxies. Webb’s Mid-Infrared Instrument (MIRI) covers the wavelength range from 5 to 28 microns, its sensitive detectors collecting the redshifted light of distant galaxies. Conveniently for us, infrared passes more cleanly through deep space gas and dust clouds, revealing the objects behind them; for this and many other reasons, the infrared spectrum has gained a crucial foothold in our cosmic investigations. Earth-orbiting satellites like NASA’s Wide Field Infrared Survey Telescope ( WFIRST ) observe deep space via longer infrared wavelengths, too.

Yet, when stars first form, they mostly issue ultraviolet light . So why don’t we use ultraviolet detectors to find distant galaxies? It’s because the universe has been stretching since its beginning, and the light that travels through it has been stretching, too; every planet, star, and galaxy continually moves away from everything else. By the time light from GLASS-z13—formed 300 million years after the Big Bang—reaches our telescopes, it has been traveling for more than 13 billion years , a vast distance all the way from a younger universe. The light may have started as ultraviolet waves, but over vast scales of time and space, it ended up as infrared. So, this fledgling galaxy appears as a red dot to NIRCam. We are gazing back in time at a galaxy that is rushing away from us.

Radio Waves

If we could see the night sky only through radio waves, we would notice swaths of supernovae , pulsars, quasars, and gassy star-forming regions instead of the usual pinprick fairy lights of stars and planets.

Tools like the Arecibo Observatory in Puerto Rico can do the job our eyes can’t: detect some of the longest electromagnetic waves in the universe. Radio waves are typically the length of a football field, but they can be even longer than our planet’s diameter. Though the 1,000-foot-wide dish at Arecibo collapsed in 2020 due to structural problems, other large telescopes carry on the work of looking at radio waves from space. Large radio telescopes are special because they actually employ many smaller dishes, integrating their data to produce a really sharp image.

Unlike optical astronomy, ground-based radio telescopes don’t need to contend with clouds and rain. They can make out the composition, structure, and motion of planets and stars no matter the weather. However, the dishes of radio telescopes need to be much larger than optical ones to generate a comparable image, since radio waves are so long. The Parkes Observatory’s dish is 64 meters wide, but its imaging is comparable to a small backyard optical telescope, according to NASA .

Eight different radio telescopes all over the world coordinated their observations for the Event Horizon Telescope in 2019 to put together the eye-opening image of a black hole in the heart of the M87 galaxy (above).

Ultraviolet Waves

You may be most familiar with ultraviolet, or UV rays, in warnings to use sunscreen . The sun is our greatest local emitter of these higher-frequency, shorter wavelengths just beyond the human visible spectrum, ranging from 100 to 400 nanometers. The Hubble Space Telescope has been our main instrument for observing UV light from space, including young stars forming in Spiral Galaxy NGC 3627, the auroras of Jupiter, and a giant cloud of hydrogen evaporating from an exoplanet that is reacting to its star’s extreme radiation.

Our sun and other stars emit a full range of UV light, telling astronomers how relatively hot or cool they are according to the subdivisions of ultraviolet radiation: near ultraviolet, middle ultraviolet, far ultraviolet, and extreme ultraviolet. Applying a false-color visible light composite lets us see with our own eyes the differences in a star’s gas temperatures.

Hubble’s Wide Field Camera 3 (WFC3) breaks down ultraviolet light into specific present colors with filters. “Science visuals developers assign primary colors and reconstruct the data into a picture our eyes can clearly identify,” according to the Hubble website . Using image-processing software, astronomers and even amateur enthusiasts can turn the UV data into images that are not only beautiful, but also informative.

X-Ray Light

Since 1999, the orbiting Chandra X-Ray Observatory is the most sensitive radio telescope ever built. During one observation that lasted a few hours, its X-ray vision saw only four photons from a galaxy 240 million light-years away, but it was enough to ascertain a novel type of exploding star . The observatory, located 86,500 miles above Earth, can produce detailed, full-color images of hot X-ray-emitting objects, like supernovas, clusters of galaxies and gases, and jets of energy surrounding black holes that are millions of degrees Celsius. It can also measure the intensity of an individual X-ray wavelength, which ranges from just 0.01 to 10 nanometers. Its four sensitive mirrors pick up energetic photons and then electronic detectors at the end of a 30-foot optical apparatus focus the beams of X-rays.

Closer to home, the Aurora Borealis at the poles emits X-rays too. And down on Earth, this high-frequency, low-wavelength light passes easily through the soft tissue of our bodies, but not our bones, yielding stellar X-ray images of our skeletons and teeth.

Visible Light

Visible color gives astronomers essential clues to a whole world of information about a star, including temperature, distance, mass, and chemical composition. The Hubble Telescope, perched 340 miles above our planet, has been a major source of visible light images of the cosmos since 1990.

Hotter objects, like young stars, radiate energy at shorter wavelengths of light; that’s why younger stars at temperatures up to 12,000 degrees Celsius, like the star Rigel, look blue to us. Astronomers can also tell the mass of a star from its color. Because mass corresponds to temperature, observers know that hot blue stars are at least three times the mass of the sun. For instance, the extremely hot, luminous blue variable star Eta Carina’s bulk is 150 times the mass of our sun, and it radiates 1,000,000 times our sun’s energy.

Our comparatively older, dimmer sun is about 5,500 degrees Celsius, so it appears yellow. At the other end of the scale, the old star Betelgeuse has been blowing off its outer layer for the past few years, and it looks red because it’s only about 3,000 degrees Celsius.

A View of Earth

Scientists use different wavelengths of light to study phenomena closer to home, too.

Detectors in orbit can distinguish between geophysical and environmental features on Earth’s changing surface, such as volcanic action. For example, infrared light used alongside visible light detection reveals areas covered in snow, volcanic ash, and vegetation. The Moderate Resolution Imaging Spectroradiometer ( MODIS ) infrared instrument onboard the Aqua and Terra satellites monitors forest fire smoke and locates the source of a fire so humans don’t have to fly through smoke to evaluate the situation.

Next year, a satellite will be launched to gauge forest biomass using a special radar wavelength of about 70 centimeters that can penetrate the leafy canopy.

💡 Why is the sky blue? During the day, oxygen and nitrogen in Earth’s atmosphere scatters electromagnetic energy at the wavelengths of blue light (450–485 nanometers). At sunset, the sun’s light makes a longer journey through the atmosphere before greeting your eyes. Along the way, more of the sun’s light is scattered out of the blue spectrum and deeper into yellow and red.

Before joining Popular Mechanics , Manasee Wagh worked as a newspaper reporter, a science journalist, a tech writer, and a computer engineer. She’s always looking for ways to combine the three greatest joys in her life: science, travel, and food.

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Three ways to travel at (nearly) the speed of light.

Katy Mersmann

1) electromagnetic fields, 2) magnetic explosions, 3) wave-particle interactions.

One hundred years ago today, on May 29, 1919, measurements of a solar eclipse offered verification for Einstein’s theory of general relativity. Even before that, Einstein had developed the theory of special relativity, which revolutionized the way we understand light. To this day, it provides guidance on understanding how particles move through space — a key area of research to keep spacecraft and astronauts safe from radiation.

The theory of special relativity showed that particles of light, photons, travel through a vacuum at a constant pace of 670,616,629 miles per hour — a speed that’s immensely difficult to achieve and impossible to surpass in that environment. Yet all across space, from black holes to our near-Earth environment, particles are, in fact, being accelerated to incredible speeds, some even reaching 99.9% the speed of light.

One of NASA’s jobs is to better understand how these particles are accelerated. Studying these superfast, or relativistic, particles can ultimately help protect missions exploring the solar system, traveling to the Moon, and they can teach us more about our galactic neighborhood: A well-aimed near-light-speed particle can trip onboard electronics and too many at once could have negative radiation effects on space-faring astronauts as they travel to the Moon — or beyond.

Here are three ways that acceleration happens.

Most of the processes that accelerate particles to relativistic speeds work with electromagnetic fields — the same force that keeps magnets on your fridge. The two components, electric and magnetic fields, like two sides of the same coin, work together to whisk particles at relativistic speeds throughout the universe.

In essence, electromagnetic fields accelerate charged particles because the particles feel a force in an electromagnetic field that pushes them along, similar to how gravity pulls at objects with mass. In the right conditions, electromagnetic fields can accelerate particles at near-light-speed.

On Earth, electric fields are often specifically harnessed on smaller scales to speed up particles in laboratories. Particle accelerators, like the Large Hadron Collider and Fermilab, use pulsed electromagnetic fields to accelerate charged particles up to 99.99999896% the speed of light. At these speeds, the particles can be smashed together to produce collisions with immense amounts of energy. This allows scientists to look for elementary particles and understand what the universe was like in the very first fractions of a second after the Big Bang. 

Download related video from NASA Goddard’s Scientific Visualization Studio

Magnetic fields are everywhere in space, encircling Earth and spanning the solar system. They even guide charged particles moving through space, which spiral around the fields.

When these magnetic fields run into each other, they can become tangled. When the tension between the crossed lines becomes too great, the lines explosively snap and realign in a process known as magnetic reconnection. The rapid change in a region’s magnetic field creates electric fields, which causes all the attendant charged particles to be flung away at high speeds. Scientists suspect magnetic reconnection is one way that particles — for example, the solar wind, which is the constant stream of charged particles from the Sun — is accelerated to relativistic speeds.

Those speedy particles also create a variety of side-effects near planets.  Magnetic reconnection occurs close to us at points where the Sun’s magnetic field pushes against Earth’s magnetosphere — its protective magnetic environment. When magnetic reconnection occurs on the side of Earth facing away from the Sun, the particles can be hurled into Earth’s upper atmosphere where they spark the auroras. Magnetic reconnection is also thought to be responsible around other planets like Jupiter and Saturn, though in slightly different ways.

NASA’s Magnetospheric Multiscale spacecraft were designed and built to focus on understanding all aspects of magnetic reconnection. Using four identical spacecraft, the mission flies around Earth to catch magnetic reconnection in action. The results of the analyzed data can help scientists understand particle acceleration at relativistic speeds around Earth and across the universe.

Particles can be accelerated by interactions with electromagnetic waves, called wave-particle interactions. When electromagnetic waves collide, their fields can become compressed. Charged particles bouncing back and forth between the waves can gain energy similar to a ball bouncing between two merging walls.

These types of interactions are constantly occurring in near-Earth space and are responsible for accelerating particles to speeds that can damage electronics on spacecraft and satellites in space. NASA missions, like the Van Allen Probes , help scientists understand wave-particle interactions.

Wave-particle interactions are also thought to be responsible for accelerating some cosmic rays that originate outside our solar system. After a supernova explosion, a hot, dense shell of compressed gas called a blast wave is ejected away from the stellar core. Filled with magnetic fields and charged particles, wave-particle interactions in these bubbles can launch high-energy cosmic rays at 99.6% the speed of light. Wave-particle interactions may also be partially responsible for accelerating the solar wind and cosmic rays from the Sun.

Download this and related videos in HD formats from NASA Goddard’s Scientific Visualization Studio

By Mara Johnson-Groh NASA’s Goddard Space Flight Center , Greenbelt, Md.

Light Year Calculator

What is light year, how to calculate light years.

With this light year calculator, we aim to help you calculate the distance that light can travel in a certain amount of time . You can also check out our speed of light calculator to understand more about this topic.

We have written this article to help you understand what a light year is and how to calculate a light year using the light year formula . We will also demonstrate some examples to help you understand the light year calculation.

A light year is a unit of measurement used in astronomy to describe the distance that light travels in one year . Since light travels at a speed of approximately 186,282 miles per second (299,792,458 meters per second), a light year is a significant distance — about 5.88 trillion miles (9.46 trillion km) . Please check out our distance calculator to understand more about this topic.

The concept of a light year is important for understanding the distances involved in space exploration. Since the universe is so vast, it's often difficult to conceptualize the distances involved in astronomical measurements. However, by using a light year as a unit of measurement, scientists and astronomers can more easily compare distances between objects in space.

As the light year is a unit of measure for the distance light can travel in a year , this concept can help us to calculate the distance that light can travel in a certain time period. Hence, let's have a look at the following example:

• Source: Light
• Speed of light: 299,792,458 m/s
• Time traveled: 2 years

You can perform the calculation in three steps:

Determine the speed of light.

The speed of light is the fastest speed in the universe, and it is always a constant in a vacuum. Hence, the speed of light is 299,792,458 m/s , which is 9.46×10¹² km/year .

Compute the time that the light has traveled.

The subsequent stage involves determining the duration of time taken by the light to travel. Since we are interested in light years, we will be measuring the time in years.

To facilitate this calculation, you may use our time lapse calculator . In this specific scenario, the light has traveled for a duration of 2 years.

Calculate the distance that the light has traveled.

The final step is to calculate the total distance that the light has traveled within the time . You can calculate this answer using the speed of light formula:

distance = speed of light × time

Thus, the distance that the light can travel in 100 seconds is 9.46×10¹² km/year × 2 years = 1.892×10¹³ km

How do I calculate the distance that light travels?

You can calculate the distance light travels in three steps:

Determine the light speed .

Determine the time the light has traveled.

Apply the light year formula :

distance = light speed × time

How far light can travel in 1 second?

The light can travel 186,282 miles, or 299,792,458 meters, in 1 second . That means light can go around the Earth just over 7 times in 1 second.

Why is the concept of a light year important in astronomy?

The concept of a light year is important in astronomy because it helps scientists and astronomers more easily compare distances between objects in space and understand the vastness of the universe .

Can light years be used to measure time?

No , despite the name, you cannot use light years to measure time. They only measure distance .

Circumference

Conservation of momentum, evaporation rate, ideal egg boiling.

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I Trekked Solo for 3 Days With Just a Backpack, and These Are the 15 Things I’m Glad I Packed — From $9

They kept this avid hiker and camper safe across 25-plus miles of mountainous terrain.

We independently evaluate all recommended products and services. If you click on links we provide, we may receive compensation. Learn more .

Travel + Leisure / Madison Woiten

I am an avid backpacker who has hiked and camped all over the world for the past 20 years. While most of my trips have been with my family, this year, I decided to challenge myself on a solo multi-day hiking and camping trip in Ras Al Khaimah UAE . I signed up for the Highlander UAE , one of the toughest hiking challenges in Asia, and over three days, I trekked about 40 kilometers (25 miles), ascending and descending 2,000 meters across rugged mountainous terrain. From the outset, I recognized the importance of managing the weight of my backpack , knowing it would determine whether I could complete the hike unassisted — which was a personal goal of mine.

After many product trials, I was able to narrow my backpacking checklist down to the ideal lightweight camping gear setup; it helped keep my pack weight at the recommended 10 kilograms (23 pounds) and ensure that everything that I needed fit in just one bag. Curious to know how I did it (and what made the cut)? Keep scrolling to learn more about the lightweight backpacking gear that will now make up my packing arrangement for every camping trip going forward.

Osprey Viva 45 Women's Backpacking Backpack

Since weight was my primary concern, I wanted a pack that would be simple to adjust on the fly without too many bells and whistles. Enter: the Osprey Viva 45 Women's Backpacking Backpack. Its base weight is 4.4 pounds and it has just one main compartment with a top lid opening as well as a rain cover in case of unexpected weather, which I experienced every day on the trail. What's more, it boasts a one-size-fits-all design with dual-side compression straps, dual trekking pole loops, and front panel lash loops to attach any extra gear that you don’t need on the trail.

It is ideal for shorter trips to the backcountry and has thick padding on the shoulder straps and back frame, which gives you additional support. I found the internal hydration sleeve with the hose port very handy as I could combine the Viva 45 with an external 5-liter hydration bladder seamlessly.

REI Co-op Trailmade 2 Tent With Footprint

At just under 6 pounds, REI’s Trailmade Two-person Tent was an easy choice for my backcountry hike. It also has a very simple setup that is easy to put up and take down at the campsite, making it an ideal backpacking tent. The Trailmade also has two small internal pockets to store your gear, but you can tuck your pack under the vestibule cover if you don’t want to bring it into the tent. With its 87-inch by 50-inch frame, it has the space to sleep shoulder-to-shoulder (in case you are sharing your tent with a fellow hiker) and put some gear at your feet. It also rained the first night on the trail and the tent kept out the moisture fairly well. 

Karthika Gupta

Exped Ultra 1R Sleeping Pad

To pad or not to pad is probably one of the biggest debated topics among backpackers. If you want to shave off some weight from your pack, you might be tempted to skip bringing one. But thanks to great advances in camping gear technology and sleeping pads like the Exped Ultra series , you don’t need to. At just 13.4 ounces. (medium size), the Ultra 1R sleeping pad made it easy to keep my overall pack weight in check without sacrificing a good night's sleep at camp. The mat inflates to 3 inches, so it provides comfort and protection from rocky and uneven terrains.

What's more, the air chambers are oriented head-to-toe, so the pad becomes slightly wider when weighted. This pad has a durable, easy-to-use, one-way flat valve that provides a high volume of airflow to ensure that you can quickly inflate and deflate the pad.

REI Co-op Magma 30 Sleeping Bag

If there is one piece of advice that I can offer for those who want to try their hand at backcountry camping, it's to invest in the lightest but warmest possible sleeping bag because it can make or break your experience. Getting a good night’s sleep, especially when you have multiple days on the trail, is crucial for your overall hiking performance. REI’s Magma 30 Sleeping Bag is one of my favorite pieces of gear to date and, at an average weight of 1 pound 10 ounces, it has earned its place as my MVP backpacking sleeping bag. 

It is made with a recycled downproof shell and water-resistant, 850-fill-power goose down for a cozy and comfortable snooze. The durable water-repellent (DWR) finish on the shell and lining beads off moisture so you will not wake up in a damp bag. My favorite part is that the zipper wraps around the front of your chest so you have more range of motion to zip and unzip yourself from the bag without feeling claustrophobic. The inner stash pocket is great for safely storing personal items close by.

REI Co-op Flash TT Women’s Hiking Boots

I have been a Merrell hiking boot fan for as far back as I can remember, but let me tell you: it was love at first stride when I tried on the Flash TT Hiking boots from REI . Right off the bat, they were so lightweight (they come in at 2 pounds, 2 ounces) and felt like I was walking on air. They also have a sock-like feel thanks to the stretchy gussets on the tongue, and the knit uppers bring the flexible feel of running shoes that molds to your foot stride. These features also kept my feet from overheating because the knit uppers helped with breathability. The insoles are decently thick and offer good arch support. They came in handy when I had to climb boulders and rocks on the trail. 

One tip is to go half a size up if your trail is remotely technical. I had almost a 2,000-meter descent on the final day of my three-day hike, and my toes were slightly bruised due to the constant downward pressure. 

Smartwool Women's Performance Hike Clear Canyon Light Cushion Ankle Socks

After hiking hundreds of miles in my lifetime, I have come to realize that hiking socks are just as important as hiking shoes. They are the understated heroes of any great hike as they help support your feet while keeping away moisture and odor. My Smartwool Performance Hike Clear Canyon Light Cushion ankle socks gave me the comfort of merino wool and nylon while having appropriate mesh zones that add to the breathability. The seamless toes also enhanced their comfort and offered a better, more performance-oriented fit overall. 

Ibex Women's Merino 24-hour Short-sleeve Low Crew Tee 

The Ibex women's Merino 24-hour short-sleeved low crew tee is my go-to for spring and warm-weather hiking, and it certainly didn’t disappoint in the hot UAE climate. Merino wool is naturally moisture-wicking, thermoregulating, odor-resistant, and breathable, so I knew that I could get by with wearing it for multiple days. It is form-fitting and soft, making it a great base layer to be paired with a light jacket in the morning (and worn by itself during the day as the temperature rises).

I also appreciated the curved hemline because it helped the shirt fit more comfortably over my hips without riding up, especially with my pack on. Ibex uses a proprietary steaming process on the wool fabric, creating a cool-to-the-touch feel that's more versatile in different climates. This is one apparel item that I know I am going to reach for repeatedly.

The North Face Women’s EA Dune Sky 9-inch Tight Shorts

These women's North Face shorts are made with soft, 80 percent recycled fabric and enhanced with the brand's moisture-wicking Flash Dry technology. This makes them ideal for hiking and other outdoor activities, especially in warm weather. After hiking 13 kilometers on the first day, I appreciated the fact that my thighs were not chaffed thanks to the breathable fabric and 9-inch inseams of these tights.

They are also fitted at the waist and thighs to keep them from riding up. The non-compressed, stretchy fit made these shorts extra comfortable, especially as the day progressed and my body continued to retain water. The drop-in side thigh pockets fit my phone and a small tube of sunscreen for easy access.

Kuhl Freeflex Roll-Up Women’s Pants

Kuhl's Freeflex roll-up pants were my one pair of back-up clothing, so I am glad that I was able to pack them in. The nights in the arid mountain desert got super cold, especially on the first night on the trail when it rained; the roll-down feature of these pants saved the day. They also have a comfortable stretch and I slept in them without feeling constricted. What's more, the Kuhl pants offer sun protection and are made with a sweat-wicking fabric, allowing them to easily double as hiking pants during the day.

These pants also have a wide waistband with internal drawcord adjustment, so I could wear them in the early morning over my hiking shorts to ward off the cold. The bottom leg-opening is wide at 15.5 inches, which means that they fit comfortably over high-top hiking shoes.

Tifosi Swank Sport Sunglasses

If you worry about losing or damaging your expensive sunglasses on the trail, then get yourself a pair of the Swank Sport sunnies from Tifosi. They have shatterproof, polycarbonate glare-reducing lenses and 100 percent UVA/UVB protection from harmful sun rays. At less than 1 pound, they are perfect for all-day comfort and easily stashing in your pack. They come in over 25 different colorways and lens tints, so there are many options to choose from.

Leki Makalu Lite Cork Trekking Poles

Even if you are hiking on relatively flat terrain, a pair of trekking poles is a great piece of gear to keep handy. They help take the pressure off your knees and make for a smoother, sturdier stride, especially if you are traversing long distances. The Leki Makalu Lite Cork Trekking Poles feature adjustable lengths and are lightweight (weighing in at just 9 ounces). The cork surface absorbs sweat while allowing for maximum grip and control, something that you'll be grateful for when going downhill. Adjustable lock security skin straps can be looped around your wrists as added support and to keep the poles in place when you're in the groove (or tackling a sharp descent).

LifeStraw Peak Squeeze Water Filter System

Water was my biggest concern on my multi-day backpacking hike. While the water bladder in my pack provided most of what I needed during the day, I supplemented my hydration with the LifeStraw Peak Squeeze Water Filter System. I felt very safe knowing that the water I was refilling was clean as the micron filter removes 99.9 percent of all bacteria and microplastics along with silt, sand, and cloudiness.

The membrane microfilter also lasts up to 500 gallons before needing replacing, so I know that this one is coming with me on all of my hikes going forward. I hooked the squeeze bottle to the easily accessible mesh pocket of my pack for hassle-free accessibility, eliminating the need to stop or unload the pack to grab the water bottle.

Primus Essential Trail Stove Kit

The Primus Essential Trail Stove Kit has been my go-to camp kitchen setup for years, even when I hike with my family or in a group. It is lightweight and, more importantly, compact in that all of the different parts of the stove kit stack up and fit neatly when not in use. It is an ideal backcountry stove and cooking kit for one to two people and includes a stove, a 0.5-liter aluminum pot, and a skillet all-in-one. You'll just need to buy the fuel separately if you are flying internationally with this kit.

CamelBak Horizon 12-ounce Camp Mug

While I can make do without a lot of creature comforts on the trail, one thing that I can't give up is my morning cup of coffee. Thankfully, I remembered to pack my CamelBak Horizon Camp Mug and clipped it to one of the many loops outside my backpack for easy access. On both days of the hike, I passed by many fellow hikers who were taking a coffee break and were only too happy to share some with me. It also comes in handy as a makeshift bowl when needed.  

Black Diamond LiteWire Carabiner

I have been using carabiners for my camping, hiking, and overall travel gear for years now. They are a great inexpensive tool to clip those little odds and ends items to your bag or camping pack securely without any second thought. I used them to clip my coffee mug, my jacket, and even my hiking bib number on the outside of my pack for easy identification. Having them handy was super convenient and helped give me hands-free access to all of the things that make it out of the pack during the hike.

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Solar eclipse 2024: A traveller’s guide to the best places to be when the light goes out

O n 8 April 2024, a total solar eclipse will sweep across North America , providing an astronomical experience in many alluring locations.

Only a tiny proportion of humanity has ever witnessed a total eclipse – but tens of millions of people will be able to experience one as the “path of totality” sweeps from the Pacific to the Atlantic during the course of that magical Monday.

Here’s what you need to know about why you should see it and where to be.

What happens during a total solar eclipse?

The greatest show on earth comes courtesy of the lifeless moon. Normally the orbiting lunar lump merely provides earth with tides, moonlight and somewhere to aim space rockets. But roughly once a year the natural satellite aligns with the sun and, thanks to a geometric miracle, blots out the hub of the solar system to create a total eclipse.

“Even though the moon is 400 times smaller than the sun, it’s also about 400 times closer to earth than the sun is,” says Nasa. “This means that from earth, the moon and the sun appear to be roughly the same size in the sky.”

A narrow band marking the “path of totality” carves an arc of darkness across the surface of our planet. If you are somewhere on that line at the predicted time, and you have clear skies, then the experience will become a lifelong memory.

The closer you are to the centre of the path of totality, the longer the total eclipse will last. The astronomer Dr John Mason, who has guided dozens of eclipse trips (and will be doing so again in 2024), says: “People down in southwest Texas will get about four minutes 20 seconds, and that reduces to about three minutes 20 seconds up in the northeast. That’s a pretty good, long total eclipse.”

What’s so good about seeing an eclipse?

In the days leading up to the eclipse, locations in the path of totality acquire something of a carnival atmosphere as astronomical tourists converge in excited anticipation.

On the day, the cosmological performance begins with a warm-up lasting more than an hour, during which the moon steadily nibbles away at the surface of the sun.

Suddenly, you experience totality. The stars and planets appear in the middle of the day. The air chills.

To testify to the heavenly fit between our two most familiar heavenly bodies, faint diamonds known as Baily’s beads peek out from behind the moon. They actually comprise light from the sun slipping through lunar valleys.

A sight to behold – so long as you can see the moon blotting out the sun and appreciate the mathematical perfection of nature in our corner of the galaxy.

Eclipses are entirely predictable: we know the stripes that the next few dozen will paint upon the surface of the Earth. But the weather is not. Cloud cover, which blighted the Cornwall eclipse in 1999, downgrades a cosmological marvel to an eerie daytime gloom.

Almost as predictable as the eclipse is that traffic towards the path of totality will be heavy on the morning of 8 April 2024.

Accommodation rates are astronomical: even humdrum motel rooms in Niagara, central in the path of totality, are selling for C$600 (£350) for the night of 7-8 April 2024.

Where will the great American eclipse 2024 be visible?

The path of totality makes landfall from the Pacific at Mazatlan on Mexico’s Pacific Coast and sweeps northeastwards to reach the US-Mexican border at Piedras Negras.

In the US, three big Texan cities – San Antonio, Austin and Dallas – are on the extremes of the path of totality; many citizens are likely to drive to locations near the centre of the line.

Arkansas will be an attractive place to see the eclipse , with both Texarkana (on the border with Texas) and Little Rock within the path of totality.

In the Midwest, Indianapolis and Cleveland share the distinction of being fairly central in the path of totality. In upstate New York, Buffalo and nearby Niagara Falls (shared with Canada) could be extremely attractive – though prone in early April to cloudy skies.

In Canada , Montreal is just touched by the path of totality. The line then reverts to the US, passing across northern Maine – which promises to be a superb with clear skies. Then back to Canada’s Maritime Provinces, with New Brunswick, Prince Edward Island and Newfoundland all in the line of darkness.

Will I be able to see a partial eclipse from the UK?

Yes. The eclipse ends with the sunset in the eastern Atlantic, about 600 miles off the coast of Cornwall , before it reaches the UK and Ireland . But on the island of Ireland and western parts of Great Britain, a partial eclipse may be visible with the sun low in the sky.

If skies are clear and you have an open view to the west, it will start at around 7.55pm in Cardiff, Liverpool , Manchester, Edinburgh and Glasgow.

BBC Weather presenter Simon King said: “With the partial solar eclipse occurring late in the day UK time, the Sun will be low to the horizon and will actually set before the spectacle is over.”

Can I combine an exciting city with a partial eclipse?

Boston, New York and Chicago are among the big cities that will see a sizeable chunk of the sun blotted out. Viewer as far apart as Alaska and the far north of Colombia and the Caribbean will, if skies are clear and they use the correct eye protection, see a partial eclipse. But there is nothing to compare with a total eclipse.

Eclipse guru Dr Mason sums up the difference between a 99 per cent partial eclipse and a total eclipse as far apart as “a peck on the cheek and a night of passion”.

“There will be people who will look at the map and say, ‘I live in Cincinnati or I live in Columbus [Ohio] and I’m just outside the zone of totality. But I’m going to get a 99 per cent-plus eclipse, so maybe I won’t bother to travel’.

“What they don’t realise is there an enormous difference between 99 per cent and 100 per cent. And there’s a range of phenomena that they won’t see if they put up with 99 per cent.”

You must use special eclipse safety glasses or viewers when viewing a partial eclipse or during the partial phases of a total solar eclipse.

Where should I be for the total experience?

There are no guarantees of clear skies: all you can do is play the odds based on the record of cloud cover for the corresponding date in previous years.

Dr Mason says the average expected cloud cover amounts increase from around 40-45 per cent on the Mexico/Texas border to over 80 per cent in Maine, New Brunswick and Newfoundland.

Three particularly tempting locations:

• Southern Texas , close to San Antonio or Austin. Besides clear skies being more likely than not, access is easy with direct flights to Austin. Importantly there is much to explore in the region before and after the eclipse, from Big Bend National Park on the Rio Grande to Space Center Houston – an excellent place to continue the cosmological theme.
• Northern Arkansas , a picturesque part of the state, with the added attraction of Memphis just a couple of hours away.
• Niagara Falls : the dramatic border between the US and Canada could be an eclipse washout due to clouds. But the natural surroundings are impeccable – and there is plenty of accommodation, which will avoid the risk of being caught in severe traffic congestion on the freeways from Toronto and locations in New York State.

However, the most recent forecasts for cloud cover suggest that the Midwest around Indianapolis and the northeastern state of Maine could have the best prospects.

When are the next total solar eclipses?

Summer 2026 – Wednesday 12 August, to be precise – should bring a spectacular eclipse visible in northern Spain at the height of the European holiday season. The path of totality begins in the Arctic and crosses Greenland and Iceland before arriving in the northern half of Spain. The stripe of darkness will traverse the great cities of Bilbao, Zaragoza and Valencia in mainland Spain before arriving in Palma de Mallorca.

The following summer (2 August 2027), the southern tip of mainland Spain is in the path of totality for an eclipse that will sweep across North Africa and the Arabian peninsula : going east from the Strait of Gibraltar, it will encompass Morocco, Algeria, Tunisia, Libya, Egypt, the northeasternmost corner of Sudan, Saudi Arabia and Yemen.

Just under 12 months later, on 22 July 2028, Outback Australia will be the place to be. A total eclipse will make landfall in northern Western Australia, sweep across the Northern Territory and part of southwest Queensland – then clean across New South Wales, with Sydney in the middle of the path of totality.

Winter cloud cover could disrupt the experience in Australia’s largest city – and is very likely in the southern portion of New Zealand’s South Island where the eclipse reaches a finale.

Australia also features in the cosmological plans on 25 November 2030. This is early summer in the southern hemisphere, and likely to be good conditions for viewing in Namibia, Botswana and South Africa (Durban is on the path of totality) as well as South Australia.

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I visit Universal Orlando several times a year. Here are my top 10 tips for first-timers.

I’ll never forget walking out of Universal Islands of Adventure in 2022, drenched from head to toe after riding Popeye & Bluto's Bilge-Rat Barge . 

It was my first trip back to Universal Orlando Resort in years, and I learned some things the hard way, which I gladly shared with fellow travelers, hoping that they’d avoid my mistakes .

Since then, I’ve had the opportunity to visit the Florida resort several times a year. And while I stand by my earlier tips, I’ve learned a few others.

Here are my top tips for first-timers visiting Universal Orlando.

1.  Stay on property

From RV parks to vacation rentals, there’s no shortage of places to stay in Central Florida, but it can pay to stay on property when visiting Universal Orlando. Not only are prices comparable to off-property hotels with Endless Summer Surfside and Dockside starting at $99 a night, but all of Universal’s resort hotels offer 30 minutes of early entry to select parks, which vary depending on time of year. 

They also provide free resort transportation, so you can get back to your room fairly quickly after a long park day or for a midday break. That free transportation can be clutch in the mornings, particularly at pricier Loews Sapphire Falls , Loews Royal Pacific , Loews Portofino Bay , and Hard Rock Hotel , which offer water taxis that drop you off closer to the parks than you can get arriving by car or bus. Cabana Bay Beach Resort and Aventura Hotel guests can walk over to Sapphire Falls for water taxis, but the extra time and steps added may make it not worth skipping the resort shuttle bus.

Walking paths are also available to the parks from every resort hotel except the Endless Summer ones. Cabana Bay guests also have a dedicated walking path to Universal’s Volcano Bay water park.

Guests staying at Royal Pacific, Portofino Bay and Hard Rock also get free Universal Express Passes , which dramatically cut waits for most rides and normally start at $79.99 per person. Free package delivery is also available to all resort hotels, if guests don’t want to lug park purchases around all day.

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2. Buy a popcorn bucket

Snacking at the parks can get expensive. Guests are welcome to bring their own snacks, but there’s one crowd-pleaser that won’t break the bank. 

A plain popcorn bucket costs a little over $12, and you can keep getting refills throughout your trip. Refills cost around $2 each, making popcorn among the cheapest, most shareable snacks on property. On our last trip, my family and I got multiple refills for the price of one pretzel with cheese dip.

3.  Bring a light, refillable water bottle

Staying hydrated is always important, particularly on hot park days. 

You can fill water bottles with free ice-cold water at any of the many Coca-Cola Freestyle machines across the property. Cups of ice water are also available for free anywhere dispensed soft drinks are sold.

Leave big double-walled stainless steel tumblers at home. Not only will they feel heavy, carrying them around all the day, but if they aren’t leak-proof, they’ll make a mess when you tip them on their sides to fit in lockers for thrill rides.

4. Bring a medium-sized bag to carry your stuff

Numerous attractions, from Jurassic World VelociCoaster to Men in Black Alien Attack , require guests to store loose articles in lockers provided for the duration of the ride. 

I previously recommended keeping most of your things together to avoid having to empty every pocket each time. That’s still true, but I want to add a caveat on size. 

The free lockers aren’t very big. They can fit a Loungefly-style mini backpack or empty popcorn bucket, if you squish them down, but they’re not large enough for packed full-size backpacks. Keep that in mind unless you want to pay for larger lockers.

5. Bring ponchos

Ponchos don’t take a lot of space and can save you from getting soaked like me on Popeye & Bluto's Bilge-Rat Barge, Dudley Do-Right’s Rip Saw Falls , and Jurassic Park River Adventure .

Even if you skip water rides, ponchos are good to keep on hand for rainy park days.

6. Use single-rider lines 

Many attractions have a single-rider lane. If you’re traveling alone or don’t mind your party being split up, you can save time using the single-rider lane, if it’s open. They’re not always offered.

It doesn’t guarantee a short wait, but it’s usually shorter. On our latest trip, my middle schooler still waited at least 45 minutes for The Incredible Hulk Coaster and an hour for Hollywood Rip Ride Rockit . That was about as long as the regular standby wait for Hulk but shorter than the standby wait for Rip Ride Rockit.

The following attractions have single-rider lanes, though availability varies.

Universal Studios Florida

• E.T. Adventure
• Harry Potter and the Escape from Gringotts
• Hollywood Rip Ride Rockit
• Men in Black Alien Attack
• Revenge of the Mummy
• The Simpsons Ride
• Transformers: The Ride-3D

Universal Islands of Adventure

• Doctor Doom's Fearfall
• Dudley Do-Right's Ripsaw Falls
• Hagrid's Magical Creatures Motorbike Adventure
• Harry Potter and the Forbidden Journey
• Jurassic Park River Adventure
• Jurassic World VelociCoaster
• The Amazing Adventures of Spider-Man
• The Incredible Hulk Coaster

7. Use child swap 

If someone in your party doesn’t meet ride requirements for an attraction or simply wants to sit it out, you can ask for a child swap. It’s particularly helpful for families with young children because it allows parents and caregivers to take turns riding the ride and staying with whoever is sitting it out.

On our most recent trip, my youngest didn’t want to ride Hagrid’s Magical Creatures Motorbike Adventure , so we all waited in line together, but then she and I peeled off to a designated waiting room while my middle schooler and mother-in-law went on the ride. When they were done, my mother-in-law stayed with my youngest while I went on the ride. My middle schooler lucked out, getting to ride it twice.

Not all attractions have waiting rooms, but they all offer swaps. Just let team members know you need one.

8. Take breaks

Theme parks can be exhausting. Not only are you on your feet all day, but they can be loud, crowded and overstimulating. 

When possible, I like to plan for at least one sit-down meal in the day. This way, I can take my time and recuperate in air conditioning away from crowds. I’ll typically splurge on one table-service meal, but there are several medium-priced, quick-service restaurants across both theme parks where you can grab a lighter snack and relax.

Really, any cool spot where you can sit down works. Knockturn Alley in The Wizarding World of Harry Potter - Diagon Alley at Universal Studios Florida is always refreshing, though dark. Jurassic Park Discovery Center in Islands of Adventure is a great place for kids to explore while adults unwind. 

9.  Watch the shows

Rides get a lot of the love, but Universal Orlando’s live entertainment is top tier. You can find showtimes on the free resort app or simply stop to watch performances you stumble across while roaming the parks.

The only year-round shows with dedicated seating are at Universal Studios Florida: Animal Actors On Location! , The Bourne Stuntacular , and Universal Orlando’s Horror Make-Up Show, which is more funny than scary.

10. Let little ones stretch their legs

There are several play areas across both theme parks for little ones to burn off energy.

This summer, a new DreamWorks Land will open at Universal Studios Florida with a “Kung Fu Panda”-themed play area.

At Islands of Adventure, kids can explore Camp Jurassic in Jurassic Park, If I Ran a Zoo in Seuss Landing, and Me Ship, The Olive in Toon Lagoon. There are also splash pads and water features across Toon Lagoon; just make sure to either bring swimsuits or backup clothes for those.

Bonus: Don’t sleep on Volcano Bay

You’ll definitely want to pack swimsuits if you’re heading to Volcano Bay, which regularly ranks among the best water parks in the country and adds a completely different experience to vacations.

It’s themed like a tropical paradise, with plenty of trees and an iconic volcano that houses multiple water slides. The water park can get crowded, but you never spend too much time standing in line because every guest gets a Tapu Tapu wearable bracelet that lets them join one virtual queue at a time. 

Single-day tickets start at $80, which is considerably less than the $119 starting price for guests age 10 and up at Universal Studios Florida or Islands of Adventure, but still not cheap. However, if you’re already planning to buy multi-day tickets for both theme parks, tacking on Volcano Bay is not much more.