light travel examples

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 BC, 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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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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NOTIFICATIONS

Light basics.

• + Create new collection

Light is a form of energy produced by a light source. Light is made of photons that travel very fast. Photons of light behave like both waves and particles.

Light sources

Something that produces light is called a light source. There are two main kinds of light sources:

Fireworks show how light travels faster than sound. We see the light almost instantly, but the sound arrives later. To work out how many kilometres away the fireworks are, count the seconds until you hear the bang and divide by 3.

Incandescent sources use heat to produce light. Nearly all solids, liquids and gases will start to glow with a dull red colour once they reach a temperature of about 525 °C. At about 2300 °C, the filament in a light bulb will start to produce all of the colours of the visible spectrum, so it will look white. The Sun, stars, a flame and molten metal are all incandescent.

Luminescent sources are normally cooler and can be produced by chemical reactions, such as in a glowstick or a glow-worm. Other luminescent sources include a computer screen, fluorescent lights and LEDs.

Light travels much faster than sound

Light travels at a speed of 299,792,458 m/s (that’s nearly 300,000 km/s!). The distance around the Earth is 40,000 km, so in 1 second, light could travel seven and a half times around the world.

Sound only travels at about 330 m/s through the air, so light is nearly a million times faster than sound.

If lightning flashes 1 kilometre away from you, the light reaches you in 3 millionths of a second, which is almost instantly. The sound of the thunder takes 3 seconds to travel 1 kilometre – to work out many kilometres away lightning is, count the seconds for the thunder to arrive and divide by 3.

Lightning storms are important for converting nitrogen gas in the atmosphere through to forms that are biologically available.

Light takes about 8 minutes and 20 seconds to reach the Earth from the Sun. When we see the Sun, we are seeing what it looked like over 8 minutes ago.

Light can travel through empty space

Unlike sound, which needs a medium (like air or water) to travel through, light can travel in the vacuum of space.

Light travels in straight lines

Once light has been produced, it will keep travelling in a straight line until it hits something else.

Shadows are evidence of light travelling in straight lines. An object blocks light so that it can’t reach the surface where we see the shadow. Light fills up all of the space before it hits the object, but the whole region between the object and the surface is in shadow. Shadows don’t appear totally dark because there is still some light reaching the surface that has been reflected off other objects.

Once light has hit another surface or particles, it is then absorbed, reflected (bounces off), scattered (bounces off in all directions), refracted (direction and speed changes) or transmitted (passes straight through).

Models for light

Wave length, height and frequency

A wave can be described by its length, height (amplitude) and frequency.

Light as waves

Rainbows and prisms can split white light up into different colours. Experiments can be used to show that each of these colours has a different wavelength.

When white light shines through a prism, each colour refracts at a slightly different angle. Violet light refracts slightly more than red light. A prism can be used to show the seven colours of the spectrum that make up white light.

At the beach, the wavelength of water waves might be measured in metres, but the wavelength of light is measured in nanometres – 10 -9 (0.000,000,001) of a metre. Red light has a wavelength of nearly 700 nm (that’s 7 ten-thousandths of a millimetre) while violet light is only 400 nm (4 ten-thousandths of a millimetre).

Visible light is only a very small part of the electromagnetic spectrum – it’s just that this is the range of wavelengths our eyes can detect.

Light as particles

In 1905, Albert Einstein proposed that light is made of billions of small packets of energy that we now call photons. These photons have no mass, but each photon has a specific amount of energy that depends on its frequency (number of vibrations per second). Each photon still has a wavelength. Shorter wavelength photons have more energy.

The photoelectric effect

University of Waikato science researcher Dr Adrian Dorrington explains the photoelectric effect. He then describes how camera sensors can be designed on the basis of this effect to enable light energy to be converted into electric potential energy.

The photoelectric effect is when light can cause electrons to jump out of a metal. These experiments confirm that light is made of these massless particles called photons.

Simple explanations of some of these concepts can be found in the article Building Science Concepts: Shadows .

Nature of science

In order to understand the world we live in, scientists often use models. Sometimes, several models are needed to explain the properties and behaviours of a phenomenon. For example, to understand the behaviour of light, two models are needed. Light needs to be thought of as both waves and particles.

Useful links

Even though light doesn’t have mass, learn how it still has a tiny amount of momentum. Find out about NASA’s solar sails to power spacecraft.

Read about the LightSail project, a crowdfunded project from The Planetary Society, aiming to demonstrate that solar sailing is a viable means of propulsion for CubeSats (miniature satellites intended for low Earth orbit).

Explore solar sails more in your classroom, with this activity Solar Sails: The Future of Space Travel from the TeachEngineering website.

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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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United Arab Emirates struggles to recover after heaviest recorded rainfall ever hits desert nation

The United Arab Emirates is struggled to recover from the heaviest recorded rainfall ever to hit the desert nation, as its main airport worked to restore normal operations even as floodwater still covered portions of major highways and roads. (AP video/Malak Harb)

A man walks along a road barrier among floodwater caused by heavy rain on Sheikh Zayed Road highway in Dubai, United Arab Emirates, Thursday, April 18, 2024. The United Arab Emirates attempted to dry out Thursday from the heaviest rain the desert nation has ever recorded, a deluge that flooded out Dubai International Airport and disrupted flights through the world’s busiest airfield for international travel. (AP Photo/Christopher Pike)

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A man carries luggage through floodwater caused by heavy rain while waiting for transportation on Sheikh Zayed Road highway in Dubai, United Arab Emirates, Thursday, April 18, 2024. The United Arab Emirates attempted to dry out Thursday from the heaviest rain the desert nation has ever recorded, a deluge that flooded out Dubai International Airport and disrupted flights through the world’s busiest airfield for international travel. (AP Photo/Christopher Pike)

An abandoned vehicle stands in floodwater caused by heavy rain with the Burj Khalifa, the world’s tallest building, seen on the background, in Dubai, United Arab Emirates, Thursday, April 18, 2024. The United Arab Emirates attempted to dry out Thursday from the heaviest rain the desert nation has ever recorded, a deluge that flooded out Dubai International Airport and disrupted flights through the world’s busiest airfield for international travel. (AP Photo/Christopher Pike)

People wait for transportation amidst floodwater caused by heavy rain on Sheikh Zayed Road highway in Dubai, United Arab Emirates, Thursday, April 18, 2024. The United Arab Emirates attempted to dry out Thursday from the heaviest rain the desert nation has ever recorded, a deluge that flooded out Dubai International Airport and disrupted flights through the world’s busiest airfield for international travel. (AP Photo/Christopher Pike)

A man carries a child through floodwater caused by heavy rain while waiting for transportation on Sheikh Zayed Road highway in Dubai, United Arab Emirates, Thursday, April 18, 2024. The United Arab Emirates attempted to dry out Thursday from the heaviest rain the desert nation has ever recorded, a deluge that flooded out Dubai International Airport and disrupted flights through the world’s busiest airfield for international travel. (AP Photo/Christopher Pike)

Vehicles drive through standing floodwater caused by heavy rain on an onramp to Sheikh Zayed Road highway in Dubai, United Arab Emirates, Thursday, April 18, 2024. The United Arab Emirates attempted to dry out Thursday from the heaviest rain the desert nation has ever recorded, a deluge that flooded out Dubai International Airport and disrupted flights through the world’s busiest airfield for international travel. (AP Photo/Christopher Pike)

Vehicles drive through standing floodwater caused by heavy rain on Sheikh Zayed Road highway in Dubai, United Arab Emirates, Thursday, April 18, 2024. The United Arab Emirates attempted to dry out Thursday from the heaviest rain the desert nation has ever recorded, a deluge that flooded out Dubai International Airport and disrupted flights through the world’s busiest airfield for international travel. (AP Photo/Christopher Pike)

People wait for transportation on Sheikh Zayed Road highway in Dubai, United Arab Emirates, Thursday, April 18, 2024. The United Arab Emirates attempted to dry out Thursday from the heaviest rain the desert nation has ever recorded, a deluge that flooded out Dubai International Airport and disrupted flights through the world’s busiest airfield for international travel. (AP Photo/Christopher Pike)

People walk through floodwater caused by heavy rain while waiting for transportation on Sheikh Zayed Road highway in Dubai, United Arab Emirates, Thursday, April 18, 2024. The United Arab Emirates attempted to dry out Thursday from the heaviest rain the desert nation has ever recorded, a deluge that flooded out Dubai International Airport and disrupted flights through the world’s busiest airfield for international travel. (AP Photo/Christopher Pike)

Abandoned vehicles stand in floodwater caused by heavy rain along Sheikh Zayed Road highway in Dubai, United Arab Emirates, Thursday, April 18, 2024. The United Arab Emirates attempted to dry out Thursday from the heaviest rain the desert nation has ever recorded, a deluge that flooded out Dubai International Airport and disrupted flights through the world’s busiest airfield for international travel. (AP Photo/Christopher Pike)

An abandoned vehicle stands in floodwater caused by heavy rain in Dubai, United Arab Emirates, Thursday, April 18, 2024. The United Arab Emirates attempted to dry out Thursday from the heaviest rain the desert nation has ever recorded, a deluge that flooded out Dubai International Airport and disrupted flights through the world’s busiest airfield for international travel. (AP Photo/Christopher Pike)

A flooded street by heavy rain is seen, with the Burj Khalifa, the world’s tallest building, on the background, in Dubai, United Arab Emirates, Thursday, April 18, 2024. The United Arab Emirates attempted to dry out Thursday from the heaviest rain the desert nation has ever recorded, a deluge that flooded out Dubai International Airport and disrupted flights through the world’s busiest airfield for international travel. (AP Photo/Christopher Pike)

A man walks through standing floodwater caused by heavy rain with the Burj Khalifa, the world’s tallest building, seen in Dubai, United Arab Emirates, Thursday, April 18, 2024. The United Arab Emirates attempted to dry out Thursday from the heaviest rain the desert nation has ever recorded, a deluge that flooded out Dubai International Airport and disrupted flights through the world’s busiest airfield for international travel. (AP Photo/Christopher Pike)

A man walks through standing floodwater caused by heavy rain in Dubai, United Arab Emirates, Thursday, April 18, 2024. The United Arab Emirates attempted to dry out Thursday from the heaviest rain the desert nation has ever recorded, a deluge that flooded out Dubai International Airport and disrupted flights through the world’s busiest airfield for international travel. (AP Photo/Christopher Pike)

A group of people work to recover an abandoned vehicle taken by floodwater caused by heavy rain in Dubai, United Arab Emirates, Thursday, April 18, 2024. The United Arab Emirates attempted to dry out Thursday from the heaviest rain the desert nation has ever recorded, a deluge that flooded out Dubai International Airport and disrupted flights through the world’s busiest airfield for international travel. (AP Photo/Christopher Pike)

Vehicles drive through standing floodwater caused by heavy rain in Dubai, United Arab Emirates, Thursday, April 18, 2024. The United Arab Emirates attempted to dry out Thursday from the heaviest rain the desert nation has ever recorded, a deluge that flooded out Dubai International Airport and disrupted flights through the world’s busiest airfield for international travel. (AP Photo/Christopher Pike)

A man walks through floodwater in the Mudon neighborhood in Dubai, United Arab Emirates, Thursday, April 18, 2024. The United Arab Emirates attempted to dry out Thursday from the heaviest rain the desert nation has ever recorded — a deluge that flooded out Dubai International Airport and disrupted flights through the world’s busiest airfield for international travel. (AP Photo/Jon Gambrell)

Civil defense officials bring water on a raft to a family in the Mudon neighborhood in Dubai, United Arab Emirates, Thursday, April 18, 2024. The United Arab Emirates attempted to dry out Thursday from the heaviest rain the desert nation has ever recorded — a deluge that flooded out Dubai International Airport and disrupted flights through the world’s busiest airfield for international travel. (AP Photo/Jon Gambrell)

Dubai civil defense officials drive through floodwater in the Mudon neighborhood in Dubai, United Arab Emirates, Thursday, April 18, 2024. The United Arab Emirates attempted to dry out Thursday from the heaviest rain the desert nation has ever recorded — a deluge that flooded out Dubai International Airport and disrupted flights through the world’s busiest airfield for international travel. (AP Photo/Jon Gambrell)

DUBAI, United Arab Emirates (AP) — The United Arab Emirates tried to wring itself out Thursday after the heaviest recorded rainfall ever to hit the desert nation , with its main airport allowing more flights even as floodwater still covered portions of major highways and communities.

Dubai International Airport, the world’s busiest for international travel , allowed global carriers on Thursday morning to again fly into Terminal 1 at the airfield. And long-haul carrier Emirates, crucial to East-West travel, began allowing local passengers to arrive at Terminal 3, their base of operations.

However, Dubai Airports CEO Paul Griffiths said in an interview with The Associated Press that the airfield needed at least another 24 hours to resume operations close to its usual schedule. Meanwhile, one desert community in Dubai saw floodwaters continue to rise Thursday to as much as 1 meter (3 feet) as civil defense officials struggled to pump out the water.

“We were looking at the radar thinking, ‘Goodness, if this hits, then it’s going to be cataclysmic,’” Griffiths said of the storm. “And indeed it was.”

The airport ended up needing 22 tankers with vacuum pumps to get water off its grounds. Griffiths acknowledged that taxiways flooded during the rains, though the airport’s runways remained free of water to safely operate. Online videos of a FlyDubai flight landing with its reverse thrust spraying out water caught the world’s attention.

“It looks dramatic, but it actually isn’t that dramatic,” Griffiths said.

Emirates, whose operations had been struggling since the storm Tuesday, had stopped travelers flying out of the UAE from checking into their flights as they tried to move out connecting passengers. Pilots and flight crews also had a hard time reaching the airport given the water on roadways.

But on Thursday, Emirates lifted that order to allow customers into the airport. That saw some 2,000 people come into Terminal 3, again sparking long lines, Griffiths said.

Others who arrived at the airport described hourslong waits to get their baggage, with some just giving up to head home or to whatever hotel would have them.

Two men walk through floodwater in Dubai, United Arab Emirates, Wednesday, April 17, 2024. (AP Photo/Jon Gambrell)

The UAE, a hereditarily ruled, autocratic nation on the Arabian Peninsula, typically sees little rainfall in its arid desert climate. However, a massive storm forecasters had been warning about for days blew through the country’s seven sheikhdoms.

By the end of Tuesday, more than 142 millimeters (5.59 inches) of rainfall had soaked Dubai over 24 hours. An average year sees 94.7 millimeters (3.73 inches) of rain at Dubai International Airport. Other areas of the country saw even more precipitation.

Meanwhile, intense floods also have struck neighboring Oman in recent days. Authorities on Thursday raised the death toll from those storms to at least 21 killed.

The UAE’s drainage systems quickly became overwhelmed Tuesday, flooding out neighborhoods, business districts and even portions of the 12-lane Sheikh Zayed Road highway running through Dubai.

The state-run WAM news agency called the rain “a historic weather event” that surpassed “anything documented since the start of data collection in 1949.”

In a message to the nation late Wednesday, Emirati leader Sheikh Mohammed bin Zayed Al Nahyan, the ruler of Abu Dhabi, said authorities would “quickly work on studying the condition of infrastructure throughout the UAE and to limit the damage caused.”

On Thursday, people waded through oil-slicked floodwater to reach cars earlier abandoned, checking to see if their engines still ran. Tanker trucks with vacuums began reaching some areas outside of Dubai’s downtown core for the first time as well. Schools remain closed until next week.

Vehicles sit abandoned in floodwater covering a major road in Dubai, United Arab Emirates, April 17, 2024. (AP Photo/Jon Gambrell)

Authorities have offered no overall damage or injury information from the floods, which killed at least one person.

However, at least one community saw the effects of the rainfall only get worse Thursday. Mudon, a development by the state-owned Dubai Properties, saw flooding in one neighborhood reach as much as 1 meter. Civil defense workers tried to pump the water out, but it was a struggle as people waded through the floodwater.

Residents of Mudon, who spoke to the AP on condition of anonymity given the UAE’s strict laws governing speech, described putting together the equivalent of nearly $2,000 to get a tanker to the community Wednesday. They alleged the developers did nothing to help prior to that, even as they called and emailed. They also said a nearby sewage processing facility failed, bringing more water into their homes.

“A lot of people were in denial of how bad it was,” one homeowner said as civil defense officials waded through the water, bringing bottled water on a raft.

Dubai Holding, a state-owned company that has Dubai Properties as an arm, did not respond to questions. It’s part of a wider nexus that U.S. diplomats have called “Dubai Inc.” — all properties overseen by the city-state’s ruling family.

The flooding sparked speculation that the UAE’s aggressive campaign of cloud seeding — flying small planes through clouds dispersing chemicals aimed at getting rain to fall — may have contributed to the deluge. But experts said the storm systems that produced the rain were forecast well in advance and that cloud seeding alone would not have caused such flooding.

Scientists also say climate change is responsible for more intense and more frequent extreme storms, droughts, floods and wildfires around the world. Dubai hosted the United Nations’ COP28 climate talks just last year.

Abu Dhabi’s state-linked newspaper The National in an editorial Thursday described the heavy rains as a warning to countries in the wider Persian Gulf region to “climate-proof their futures.”

“The scale of this task is more daunting than it appears even at first glance, because such changes involve changing the urban environment of a region that for as long as it has been inhabited, has experienced little but heat and sand,” the newspaper said.