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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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How Does Light Travel?

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 era, the 20th century led to breakthroughs that showed us 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 unanswered questions about 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 to 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 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 divert it, or arrest it, 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!

We have written many articles about light here at Universe Today. For example, here’s How Fast is the Speed of Light? , How Far is a Light Year? , What is Einstein’s Theory of Relativity?

If you’d like more info on light, check out these articles from The Physics Hypertextbook and NASA’s Mission Science page.

We’ve also recorded an entire episode of Astronomy Cast all about Interstellar Travel. Listen here, Episode 145: Interstellar Travel .

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56 Replies to “How Does Light Travel?”

“HOW DOES LIGHT TRAVEL?”

it travels lightly. 😀

Light doesn’t exist. This is an observation from light’s point of view and not ours. Traveling at the speed of (wait for it) light, absolutely no time passes between leaving it’s source and reaching it’s destination for the photon. This means, to the photon hitting your retina, it is also still on that star you are observing 10 light years away. How is this possible? Maybe John Wheeler was right when he told Richard Feynman that there is only one electron in the universe and it travels forward in time as an electron, then back in time as a positron and every electron we see is the same electron.

MY QUESTION IS: Whether light is a wave , particle or both.. where does it get the energy to move through space/time. In other words is the energy of light infinite? Does it continue on without lose of energy…..forever…….

I believe that Special Relativity says that the energy of light is infinite due to the very fact it has no mass. E=MC^2

In reverse, this is also why something with mass to begin with. If accelerated toward the speed of light, will see their mass and gravity increase to infinite points as they near relativistic speed (it actually starts around 95% with a steep upward curve from there), with a relative slowing to a stop of time.

Join the discussion

Light and the universe are only illusions that are formed in our minds via technology that sends information from the simulation program we’re living in. That information comes in the form of invisible wavelengths that includes wavelengths that we perceive as light. The visible retinas in our eyes are like tiny video screens where these particles are arranged into patterns that form into all the various objects we think are real objects. This information is also converted into thoughts within our minds which are like computer processors that process that information.

We are living in a computer simulation that is much more advanced than anything the characters in the program have built according to the information called the Beast.

Brad,…So You’re suggesting that “life” as we know and call it “is some kind of retro-virus” or “bio-intelligent format” heaped upon a perceived “set of accepted data sets” that are not in sync with each other in most cases with exception to Math 94% of the time….Even then it can vary which suggests Your idea would mean we all live in a fairy tale. That is what you suggest,…right?……

Brad has watched the Matrix too many times.

Correction: Even gravity doesn’t slow light down. Light (EM radiation of any wavelength) always travels at speed c, relative to any local inertial (Lorentz) frame. It could also be noted that the wavelength of an EM wave is not a characteristic of that wave alone; it also depends on the state of motion of the observer. You might even say, “One man’s radio wave is another man’s gamma ray.”

Light actually “slows down” every time it has to travel through anything but a vacuum. Look up Cherenkov radiation to see what happens when light initially travels faster than it can through a particular substance, like water. Light speed is not constant when traveling through any medium except pure vacuum. In fact that is why your pencil looks bent when you drop it in a glass of water. Light bends to find it’s fastest path through any medium, and it slows down in that medium.

if all you scientist could ever get it in your pie brain that there is no time, no light speed, no warping space, no black holes for the purpose of moving through space quickly, no smallest no biggest when it comes to space and that all of everything has always been in existence but not necessarily as it is now. you will never find the smallest because if it exist it has an inside, and you will never find the end of space because it is infinite.

What are you smoking?

The article started out nicely, but I lost interest as mistakes began to appear. First Einstein did not “propose” the photoelectric effect. The photoelectric effect was first observed by Heinrich Hertz in 1887. Einstein used the idea of photons to explain the photoelectric effect and derive the photoelectric equation. Also, Max Plank had already derived the blackbody distribution, by assuming that electromagnetic energy of frequency f could only be emitted in multiples of energy E=hf, by 1900. Einstein’s paper on the photoelectric effect was published in his “miracle” year of 1905. The photoelectric effect has nothing to do with black body radiation.

Einstein did not coin the name “photons” for light quanta, as stated in this article. This term was first used by Arthur Compton in 1928.

I have to say that I do not know what the author of the article means when he says ” calculating the wavelength at which light functioned” in reference to Louis-Victor de Broglie. Louis de Broglie used the dual nature of light to suggest that electrons, previously thought of as particles, also had wave characteristics and used this notion to explain the Bohr orbits in the hydrogen atom.

I gave up on the article after seeing these errors. I’m afraid I have a low tolerance for sloppy writing.

Oh, it’s BCE now, “Before the Common Era” BC has worked for 2000 years but now the PC police have stepped in so as not to offend who? Some Muslims?

mecheng1, you must be very young. BCE has been in used in academia for decades. It’s nothing “new”, just out of your circle of knowledge.

Decades??? Really?? How does that compare to 2000 years?

Only in Euro-centric texts have your assertions been true, McCowen. The rest of the world not influenced by Christianity have used their own calendars and a “0” year or a “year 1” from which to reckon the passage of time, largely based on their own religions or celestial observations.

Over the last century or so, through commerce, most of the world has generally accepted the use of a Western calendar (or use it along with their own for domestic purposes, like we here in the US still use Imperial units of measure that have to be converted to metric for international commerce). So, we are in a “common era” insofar as non-Christian societies are incorporating the Gregorian Calendar and the generally-accepted “year 1” established by that calendar (which is supposed to be the year of Jesus’s birth, but it probably isn’t according to current scholarship). Besides, the Gregorian calendar is an improved derivative of the Roman calendar – even the names of the months come from the Romans.

In short, it is more accurate, as well as respectful, to go with BCE in these global times.

Where is the information carried on a photon hitting my eye(s), or cluster/group/pack of photons hitting my eyes(s), that I see as other distant galaxies and planets going around stars?

That’s the mystery, isn’t it? Even in scattering, light remains coherent enough to convey an enormous amount of information.

Since the miniscule equal masses with opposite charges, that make up the photon structure, interact at 90 degrees, this induces a spin (a finding from the 80’s by the LANL plasma physics program) which creates a centrifugal force that counterbalances the charge attraction of the opposite charges. This establishes a stable structure for energies less than 1.0216 MeV, the pair-formation threshold, separating these “neutrino” sub-components by a specific distance providing wavelengths varying with photon energy. This composite photon propagates transversely at c/n, the speed of light divided by the index of refraction of the material traversed. In spite of the mass being defined as zero, for convenience in calculating atomic masses, there is actually an infinitesimal but non-zero mass for the photon that is required for calculations that describe its properties.

Tim, you poor guy! You have a discombobulated brain! Everything you wrote is just gibberish.

i would like to know the temperature in a black hole…maybe absolute zero? is absolute zero the moment that time stop?

I think the temp inside a black hole would be extremely high since temperature seems to increase with mass. Comparing absolute zero to time stopping is very interesting though. To the observer they would appear the same.

Theoretically there is no temperature in a black hole from any observer POV because time is stopped. Although JALNIN does bring up that point, and he also brings up the point of increasing mass corresponding to increasing energy. Everything in Hawking and Einstein’s equations though, suggest that any energy would be absorbed back by the singularity, so there wouldn’t be any heat. In fact it should be infinitely cold. But time is no more, so technically no heat or energy is emitted anyway from any observers POV. Yet recent images of black holes from Chandra show that they emit powerful Gamma Jets along their spin axis just like Neutron stars, and Pulsars. BTW edison. The accretion disk can reach temperatures of 20MN Kelvin on a feeding SM black hole (quasar). NASA just published an article on it through the Chandra feed a while back.

Light doesn’t travel, it just IS. It is we, the condensed matter, that travels, through time.

Oh really? Is this just your imagination/illusion or you have published a paper on it?

So you don’t believe you travel through time?

I wish I understood just a portion of I just read, love sicence so bad BUT, sighs

It would be easier to understand if it wasn’t pure gibberish written by someone with no science background.

I have two “mind-bending relativity side effects” to share. At least they are mind-bending to me.

1) Light travels the same speed relative to all particles of mass, regardless of how those particles move relative to each other:

I can conceptualize this if we are only talking about two mass-particles/observers and the examples I’ve seen always involve only two observers. But if you have many mass-particles/observers, how does the space-time seem to know to adjust differently for all of them. I am sure i am understanding this correctly as it is a basic concept of special relativity and nobody seems to bring this issue up. But it “bends my mind” when i try to include more than two observers. Maybe you can help.

2) General Relativity’s (“GR”) prediction that the big bang started with “Infinite” energy and now the universe appears to have finite mass energy and Regarding the first effect: How can something infinite turn into something finite? Is the answer that at that early in the universe, quantum takes over and GR’s prediction of infinite mass-energy at the start of the universe is just wrong?

I need to correct a typo in my previous comment. Where i say “i am sure am understanding this correctly” I meant to include the word NOT. so it should read “i am sure am NOT understanding this correctly” Mark L.

Mark,….I think you’re understanding it just fine from the standpoint of multiple observers, The point might be that in space, the density of “emptiness” or “lack of emptiness” might be impacted from one area of observation to another by an observer who’s perceptions are not equal but not being taken into consideration by each observer. ( an example if I may?) If you were to use a Clear medium which is oil based beginning with 5 gallons of mineral spirits in a large barrel and keep adding 5 gallons of thicker clear oil and then heavy grease and stop with using a clear heavy wax,…what happens is you end up with a barrel of clear fluid that begins with a floating substrate but the liquid begins to keep floating and the heaviest stuff goes to the bottom,…You end up with a sort of solid tube of clear fluids which if you could keep them in shape here on the earth, “you could observe them” from several positions, #1. the fluid end #2, the less fluid part, #3, the semi solid part #4. the seemingly solid part #5. the almost solid part & #6. the solid part……all of which would be transparent….You could then shine a laser through all of it and perhaps do that again from different places and see what happens at different angles…..I think what happens as a result would be, an observer would end up be influenced as per his or her ideas thusly because of the quasi-nature of what the density of space is at the point of space is where the observation is made. just a guess.

All Special Relativity really says about light is that it appears to move at the same rate from any observer POV. There are other more advanced rules relating to light speeds. One of them is the implication of infinite energy in a photon because of the fact it’s mass-less, therefore it can move at the maximum rate a mass-less particle or wave can (not necessarily that it does) Later when the electron was discovered (also mass-less particle or wave), it was also found to conform to the rules of special relativity.

As far as the big bang, there are a lot of cracks in that theory, and many different ones are beginning to dispute some of the common ideas behind the “Big Bang” as well as “Inflationary Cosmology”. Honestly though, both standard and quantum physics applied, and yet both went out the window at the same time at some point. That’s what all the theories really say. At some point, everything we know or think we know was bunk, because the math just breaks down, and doesn’t work right anymore.

i think until there is an understanding of the actual “fabric” of space itself, the wave vs particle confusion will continue. another interesting article recently was the half integer values of rotating light. planck’s constant was broken? gravity? a bump in the data? lol these are interesting times.

There’s no fabric.

Tesla insists there is an aether, Einstein says not. Tesla enjoyed far less trial and error than Einstein. The vast majority of Tesla’s projects worked the first time around and required no development or experimentation. I’ll go with Tesla; there is an aether as a fabric of space.

http://weinsteinsletter.weebly.com/aether.html

Maybe Special Relativity is not correct? 🙂

Feynman said unequivocally that QED is NOT a wave theory. In fact, the math only looks like Maxwell’s wave function when you are looking at a single particle at a time, but the analogy breaks down as soon as you start looking at the interactions of more than one, which is the real case. There’s no light acting alone, but always an interaction between a photon and some other particle, an electron, another photon, or whatever. He said “light is particles.” So the question re: how can light travel through a vacuum if it’s waves is a nonsensical question. There are no collapsing wave functions in light. There’s only probabilities of position that look like waves on a freaking piece of paper. Even calling light properties as “wavelengths” is nonsensical. Light comes in frequencies, i.e., the number of particles traveling tightly together. Higher frequency is more energy because it’s more particles (E=MC[squared]). “Wavicles” is pure bullshit.

I don’t agree with the John Wheeler theory that there is only one electron since the computer I am using was built by ion implantation and uses a very large number of them simultaneously to function.

Black holes don’t stop or slow light, if they even exist. A black hole could phase shift light, which is why we see things emitting xrays and call them black holes….but they could be something else too.

Photons have no mass but they do have energy. Energy and mass are transformable into each other. Gravity works on energy as well as mass. As massive particles approach the speed of light their measurable mass increases to infinity. But since energy is equivalent to mass, why doesn’t the photon, which has energy, not seem to have infinite mass?

NO other wave travels thru a vacuum? what about radio?

Radio waves are a specific frequency range of light.

Technically speaking, radio waves are emitted at various frequencies that share the same space time as light. They are not however light. They’re modulated electrons. Modulated photons certainly can be used to carry a vast amount of information a great distance. It cannot do it any faster or better than a radio wave though. Both electrons and photons are mass-less, therefore they both conform to the rules of Special Relativity in the same way. Both travel at the speed of light.

I just don’t understand is it a particle of a wave? It seems like it behaves like wave and sometimes like particle and in some situations is like a what ever you are going to call it.

So, the logical idea would to have formula Photon_influence * weight_for_particle + Wave_influence * weight_for_wave

Make it more compact.

This article is good but the title is bad as by the end we still weren’t told how light travels through space. Also, there are some historical mistakes as already pointed out. Now for my contribution: I think that light and Gravity have a lot in common; for one – an atom’s electrons transmit light and an atom contains the tiny heavy place that knows everything there is to know about gravity, that is, the nucleus. Light and Gravity are both related to the same entity, the atom. Unfortunately, we, still cannot grasp how what’s heavy brings about gravitation. For those of you with a creed for new ideas go to: https://www.academia.edu/10785615/Gravity_is_emergent It’s a hypothesis…

Gravity and light are infinite, like space and time… Mind the concept that there are waves within waves, motions within motion, vibrations within vibration, endless overtones and universal harmony…

From this article, I have “And in the end, the only thing that can truly slow down or arrest the speed of light is gravity”

Doesn’t light slow down in water and glass and other mediums. I was only a Physics minor, but I do remember coivering this though way back in the early 80’s. And in my quick checking online, I found the following.

“Light travels at approximately 300,000 kilometers per second in a vacuum, which has a refractive index of 1.0, but it slows down to 225,000 kilometers per second in water (refractive index = 1.3; see Figure 1) and 200,000 kilometers per second in glass (refractive index of 1.5).”

Were they saying something else here. I did like the article.

Photons are not massless, but their mass is incredibly small even compared to a proton or neutron. So, by Einstein’s E=MC^2, the energy required for a photon to move is greatly reduced, but photons do have mass and are affected by gravity. If photons had no mass at all, then gravity would have no affect on them, but gravity does. Gravity bends light and can change it’s course through space. We see that in the actual test first performed to prove Einstein’s theory buy observing the distorted placement of stars as their light passes near the sun observed during an eclipse. We can also see it through gravitational lensing when viewing deeps space objects. And the fact that there are black holes that are black because light cannot escape it’s gravity. So photons do have mass, be it miniscule, and with that their propagation with light waves through space will eventually run out of energy and stop. but this would probably require distances greater to several widths of our universe to accomplish. Light from the furthest reaches of the universe are not as bright, or as energetic, as they are at anyplace between here and their origins. That reduction in their energy is also attributed to Einstein’s equation and the inverse square law, where the intensity of light is in relation to the inverse square of the distance. That proves that light looses energy the further it travels, but it still moves at the speed of light. As light looses energy, it doesn’t slow the light wave.

It has been proven that more energetic light does in fact travel slightly faster. You can find the experiments done with light that has traveled billions of light years, the more energetic is in fact faster over a number of seconds, around 10 -15 or so. As people encounter this information, they see that many accepted theories can now be debunked.

The point of the article is nothing new; light acts like a particle AND a beam. So when you sit behind a closed door and someone shines a light on the door, the light will engulf the door and wave through and around the edges, the particle does not just bounce straight back. You can focus a beam of light on an object, but it will sneak though the corners and underneath the door, through any opening,. And yes, light travels forever. It is a constant, that cannot be sped up. We can slow it down by focusing it through prisims or crystals. But it still is traveling at 186,000/MPS.and that speed does not change. So, that is why we can see the outer edge of the universe: 13,8B light years away *the time that it takes for light to travel in one year, is one light year. So, it has taken 13,8B light years for the light of other galaxies to get here, so those galaxies could be gone by now, since it took so long to reach us, We are truly looking back in time as we see the light emitted from those galaxies and stars.

It propagates through the quantum mish-mash know as the aether . . .

If light is a particle and particles have mass why does not the mas increase with it speed?

Wow…there are errors in the article, yes…the enthusiasm demonstrated by all the comments is encouraging…but when I read these comments, I am a bit dismayed at the lack of understanding that is evident in most of them…confusing energy and intensity and wavelength…confusing rest mass and inertial mass…not to mention some off-the-wall hypotheses with no experimental evidence to support them. There are some great primers out there…books, documentaries, podcasts (like Astronomy Cast). Good luck!

Precisely correct. Sci-fi rules basic physics, which reflects on the poor education system. Pity.

First time I heard about A. A. and his theory about light I really didn’t like him. Why? Because light was the the fastest thing in the universe and there is no other thing faster than the light. Later, when I have red about angular speed I have asked my self if you have linear and angular speed and both of them are speeds how that will result in the maximum speed. Since then, I have not had a chance to get right answer.

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How Does Light Travel Through Space and Other Media?

Light is one of the most enigmatic of entities in the universe. You are not the first person to carefully reflect upon this question about light travel. From Galileo, Newton, to Einstein, every one of the great minds has thought about this question and thanks to them, we have an answer today, in terms of the classical theory of electromagnetism.

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The Cosmic Speed Limit

According to Einstein’s special theory of relativity, the speed of light is constant in vacuum and no object can exceed it. In short, nothing can travel faster than the speed of light. It is the cosmic speed limit for information exchange.

Scientists have measured the speed of light to be 299,792,458 meters per second. It is the highest speed that can be achieved by any entity. So far, no object has been found to travel at a velocity that exceeds the speed of light.

One of the central pillars of modern physics, the special theory of relativity, will turn out to be wrong, if light speed is exceeded by any other entity. In short, light is the fastest thing in our universe and no information can travel faster than it. How does light achieve this phenomenal speed? How does it travel at all? Let us find out.

The Nature of Light: Wave/Photons

One can understand the phenomenon of light travel, once its nature is understood. The first glimpses of the nature of light were provided by the ingenuity of James Clark Maxwell. It came as a brilliant revelation to him as he was constructing a theory that describes the electric and magnetic forces between stationary and moving charged objects.

He discovered that electricity and magnetism were two sides of the same coin. His theory combined electricity and magnetism into the unified force of electromagnetism and light was found to be an electromagnetic wave. This theory pictures light as a transverse electromagnetic wave, traveling through space. This is the classical perspective.

However, light can also be alternately perceived as a particle, known as a photon, if you look at it, from the perspective of quantum mechanics, which quantizes all matter and energy. In fact, according to quantum field theory, everything behaves like a particle and a wave. Not just photons, but electrons, which were earlier thought to be particles, are now known to behave as waves. Both, particle and wave viewpoints are equivalent, and for this discussion, we will focus on the wave perspective, which provides a simpler explanation of light travel.

How Light Travels Through Space

So light is a kind of wave. However every wave, waves something. That is, every wave like a sea wave, travels through a medium, in which it causes disturbances and creates undulations.

If a sea wave is a ripple or a disturbance that travels through water, then what is the medium in which light travels as a wave? The answer is none. This is where light is different from any other kind of a wave. It can travel through vacuum and it does not require a medium to propagate.

How does light manage to do this? The answer lies in the fact that it is an electromagnetic wave, carrying energy. Maxwell discovered how light travels through vacuum. Here is an illustration of an electromagnetic wave, that illuminates its nature and clearly presents its component parts.

Here’s an explanation of what makes light travel possible, in a nutshell. Any charged object has an electric field associated with it. When that electric field changes, a changing magnetic field is created. Consequently, the changing magnetic field again creates an electric field. This continues ad infinitum, creating an electromagnetic wave, traveling in space. This is how light travels, as a disturbance or ripple in the electromagnetic field.

n short, an accelerated charge radiates electromagnetic waves. Mathematically, this relation can be understood through the solution of Maxwell’s equations for free space.

Visible light, radio waves, and even X-rays are all electromagnetic waves of different wavelengths and frequencies. The wavelength of visible light is such that our eyes are tuned to it, just like a radio receiver is tuned to radio waves. Ergo, our eyes can perceive that part of the electromagnetic spectrum.

Light travels in straight lines in vacuum. However, in a material medium, light shows two properties of reflection and refraction. When a light wave cannot penetrate an object, it gets reflected back and when a light wave travels inside a medium, it bends or gets refracted.

When light travels through a medium, it interacts with its electric field. Depending on the nature of that field, one can predict the degree of bending or refraction that light undergoes, when passing through it.

Here’s how you perceive light, through your eyes. Light entering inside the white of your eyes (cornea), through the lens, bends or refracts, to get focused on your retina. The retina converts the incident light into electric signals, from which your brain creates an image, that you perceive.

Light Travel Through Different Media

Though the speed of light is constant in vacuum, when traveling through denser media, it slows down considerably. The degree of bending or slowing down of light velocity in different media, is measured by the refractive index of a particular medium. Light undergoes bending or slowing down, when entering a denser medium, from a rarer one and speeds up when entering a rarer medium, from a denser one. Let us see what happens in both cases.

As illustrated in the diagram presented below, when light enters a denser medium, it slows down and bends towards the normal. Here, the angle of refraction (r) is lesser than the angle of incidence (r < i).

As demonstrated by the diagram presented below, when entering a rarer medium, from a denser medium, light speeds up, and bends away from the normal. Here, the angle of refraction is greater than the angle of incidence (r > i).

If the refractive index of one medium is known, the other one can be deduced from Snell’s law, if the angles of incidence (i) and refraction (r) are known. It’s stated as: sin i/sin r = n2/n1 where, ‘i’ is the angle of incidence, ‘r’ is the angle of refraction, and n1 is the refractive index of the medium of incidence, while n2 is the refractive index of the medium of refraction.

To fully understand how the dynamic nature of the electromagnetic field leads to the creation of waves, you will have to dig deep into Maxwell’s equations, presented in the graphic below. A good starting point is The Feynman Lectures on Physics, Vol.II. If God is perceived to be the laws of nature, one could say the following.

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Light basics.

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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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Dominic - This has to do with two theories of physics called quantum mechanics and special relativity. And the question that was troubling physicists about 100 years ago was whether light is a particle or a wave. The problem is, you can describe some properties of light as being like particles and some properties such as diffraction and refraction in terms of a wave-like motion. And what physicists decided was that in fact, it's both at the same time. So light is made up of particles that we call photons, but it's also made up of waves of electric and magnetic fields which are electromagnetic waves.

The question is, do those waves move through some medium like ripples on top of a pond, or do they just move through the vacuum of space?

If they're like ripples in a pond then if you travel at some speed with respect to that medium then you should see light traveling at different speeds. And experiments have determined that light travels at the speed of light, however fast you're traveling in whatever direction. And so, that suggests there is no medium through which light travels.

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How Does Light Travel Through Space? Facts & FAQ

Last Updated on Mar 15 2024

Light is such a fundamental part of our lives. From the moment we’re born, we are showered with all kinds of electromagnetic radiation, both colorful, and invisible. Light travels through the vacuum of space at 186,828 miles per second as transverse waves , outside of any material or medium, because photons—the particles that make up light—also behave as waves. This is referred to as the wave-particle duality of light.  

• What Is Light?

The wave-particle duality of light simply means that light behaves as both waves and particles . Although this has been long accepted as fact, scientists only managed to observe both these properties of light ¹ simultaneously for the first time in 2015.

As a wave, light is electromagnetic radiation—vibrations, or oscillations, of the electric and magnetic fields. As particles, light is made up of little massless packets of energy called photons ¹ .

• What Are Light Waves?

Waves are the transference of energy from one point to another. If we dropped a pebble into a small pond, the energy that the impact creates would transfer as a ripple, or a wave, that travels through the surface of the water, from one water particle to another, until eventually reaching the edge of the pond.

This is also how sound waves work—except that, with sound, it’s the pressure or vibrations of particles in the air that eventually reach our ears.

Unlike water and sound, light itself is electromagnetic radiation—or light waves—so it doesn’t need a medium to travel through.

• What Are Transverse Waves?

Light propagates through transverse waves. Transverse waves refer to a way in which energy is transferred.

Transverse waves oscillate at a 90-degree angle (or right angle) to the direction the energy is traveling in. An easy way to picture this is to imagine an S shape flipped onto its side. The waves would be going up and down, while the energy would be moving either left or right.

With light waves ¹ , there are 2 oscillations to consider. If the light wave is traveling on the X axis, then the oscillations of the electric field would be at a right angle, either along the Y or Z axes, and the oscillations of the magnetic field would be on the other.

• Can Anything Travel Faster Than Light?

The simple answer to this question is no, as far as we know at this time, nothing can go faster than the speed of light ¹ . Albert Einstein’s special theory of relativity states that “no known object can travel faster than the speed of light in a vacuum.”

Space and time don’t yet exist beyond the speed of light—if we were to travel that fast, the closer we get to the speed of light, the more our spatial dimension would shrink, until eventually collapsing.

Beyond this, the laws of physics state that as an object approaches the speed of light , its mass would become infinite, and so would the energy it would need to propel it. Since it’s probably impossible to create an infinite amount of energy, it would be difficult for anything to travel faster than light.

Tachyon, a hypothetical particle, is said to travel faster than the speed of light. However, because its speed would not be consistent with the known laws of physics, physicists believe that tachyon particles do not exist.

• Final Thoughts

Light travels through space as transverse, electromagnetic waves. Its wave-particle duality means that it behaves as both particles and waves. As far as we know, nothing in the world travels as fast as light.

• https://phys.org/news/2015-03-particle.html
• https://www.nature.com/articles/ncomms7407
• https://www.wtamu.edu/~cbaird/sq/2017/07/20/is-the-reason-that-nothing-can-go-faster-than-light-because-we-have-not-tried-hard-enough/
• https://www.physics.brocku.ca/PPLATO/h-flap/phys6_1f_1.png
• https://science.nasa.gov/ems/02_anatomy

Featured Image Credit: NASA, Unsplash

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About the Author Cheryl Regan

Cheryl is a freelance content and copywriter from the United Kingdom. Her interests include hiking and amateur astronomy but focuses her writing on gardening and photography. If she isn't writing she can be found curled up with a coffee and her pet cat.

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Why Can Light Travel Through Space: Unraveling the Mysteries of Cosmic Illumination

The Physics Behind Light’s Journey through the Cosmos

Light is a fascinating phenomenon that has puzzled scientists and thinkers for centuries. One of the most intriguing questions surrounding light is why it can travel through space. To unravel this mystery, we must delve into the realm of physics and explore the fundamental principles that govern the behavior of light.

At its core, light is an electromagnetic wave. It consists of oscillating electric and magnetic fields that propagate through space. These fields are perpendicular to each other and the direction of the wave’s motion. This unique property allows light to travel in a vacuum, such as the vast expanse of space.

The Role of Electromagnetic Waves in Space Travel

Electromagnetic waves play a crucial role in the transmission of energy through space. They are characterized by their wavelength and frequency, which determine the properties of different types of electromagnetic radiation. Light, as a form of electromagnetic radiation, falls within a specific range of wavelengths and frequencies that enable it to travel through space.

Exploring the Vacuum of Space: A Perfect Medium for Light

Space is often referred to as a vacuum, devoid of matter and air. This unique environment provides an ideal medium for light to propagate. Unlike other forms of energy, such as sound or mechanical waves, light does not require a material medium to travel. The absence of particles in space allows light to travel unimpeded, covering vast distances without encountering any resistance.

The Speed of Light: A Universal Constant that Defies Boundaries

One of the most remarkable aspects of light is its speed. The speed of light in a vacuum is approximately 299,792 kilometers per second or about 186,282 miles per second. This incredible velocity makes light one of the fastest phenomena in the universe. It allows light to traverse immense cosmic distances and reach us from distant stars and galaxies.

Why Other Forms of Energy Struggle to Travel through Space

While light can effortlessly travel through space, other forms of energy face significant challenges. Sound, for example, relies on the vibration of particles to propagate. In the absence of a material medium like air or water, sound waves cannot travel. Similarly, mechanical waves require a medium to transfer energy, making them unsuitable for space travel.

The Interplay Between Light and Dark Matter in the Universe

Dark matter is another intriguing cosmic element that interacts with light. Although invisible and mysterious, dark matter exerts a gravitational pull on surrounding matter, including light. This interaction can cause light to bend or be lensed as it travels through space. Understanding the interplay between light and dark matter is an ongoing area of research in astrophysics.

Frequently Asked Questions about why can light travel through space

Q: Why can light travel through space while other forms of energy cannot?

A: Light is an electromagnetic wave that does not require a material medium to propagate. This allows it to travel through the vacuum of space without encountering any resistance.

Q: How fast does light travel through space?

A: The speed of light in a vacuum is approximately 299,792 kilometers per second or about 186,282 miles per second.

Q: Can sound waves travel through space?

A: No, sound waves require a material medium, such as air or water, to propagate. In the vacuum of space, where there is no air or matter, sound waves cannot travel.

Q: What is the role of dark matter in the interaction of light in space?

A: Dark matter exerts a gravitational pull on surrounding matter, including light. This interaction can cause light to bend or be lensed as it travels through space.

Expert Advice

Understanding the phenomenon of why light can travel through space is a complex topic that requires a deep understanding of physics and electromagnetic waves. Scientists continue to investigate this intriguing aspect of our universe, unraveling the mysteries of cosmic illumination. The ability of light to traverse the vastness of space has allowed us to explore distant celestial objects and gain valuable insights into the workings of the cosmos.

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• light in space
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I'm a space enthusiast with a passion for sharing the wonders of the universe. With a background in Space Science, I've spent the last 4 years exploring Astrophysics, aiming to make space science accessible to everyone.

This website is my platform to share fascinating discoveries and insights about space. Whether you're deeply involved in space studies or simply curious about the stars, I hope to ignite your passion for exploration and discovery. Join me as we delve into the mysteries of the universe together!

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Science News Explores

Explainer: understanding waves and wavelengths.

Waves transfer energy, not matter, from one place to another

The swells of water in the ocean, the sunlight shining down and the sound of the crashing water ⎯ all are types of waves.

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By Jennifer Look

March 5, 2020 at 6:40 am

Waves appear in many different forms. Seismic waves shake the ground during earthquakes. Light waves travel across the universe, allowing us to see distant stars. And every sound we hear is a wave. So what do all these different waves have in common?

A wave is a disturbance that moves energy from one place to another. Only energy —  not matter — is transferred as a wave moves.

The substance that a wave moves through is called the medium . That medium moves back and forth repeatedly, returning to its original position. But the wave travels along the medium. It does not stay in one place.

Imagine holding one end of a piece of rope. If you shake it up and down, you create a wave, with the rope as your medium. When your hand moves up, you create a high point, or crest. As your hand moves down, you create a low point, or trough (TRAWF). The piece of rope touching your hand doesn’t move away from your hand. But the crests and troughs do move away from your hand as the wave travels along the rope. 

The same thing happens in other waves. If you jump in a puddle, your foot pushes on the water in one spot. This starts a small wave. The water that your foot hits moves outward, pushing on the water nearby. This movement creates empty space near your foot, pulling water back inwards. The water oscillates, moving back and forth, creating crests and troughs. The wave then ripples across the puddle. The water that splashes at the edge is a different bit of water than where your foot made contact. The energy from your jump moved across the puddle, but the matter (the molecules of water) only rocked back and forth.

Light, or electromagnetic radiation, also can be described as a wave. The energy of light travels through a medium called an electromagnetic field. This field exists everywhere in the universe. It oscillates when energy disturbs it, just like the rope moves up and down as someone shakes it. Unlike a wave in water or a sound wave in air, light waves don’t need a physical substance to travel through. They can cross empty space because their medium does not involve physical matter. 

Scientists use several properties to measure and describe all these types of waves. Wavelength is the distance from one point on a wave to an identical point on the next, such as from crest to crest or from trough to trough. Waves can come in a wide range of lengths. The wavelength for an ocean wave might be around 120 meters (394 feet). But a typical microwave oven generates waves just 0.12 meter (5 inches) long. Visible light and some other types of electromagnetic radiation have far tinier wavelengths.  

Frequency describes how many waves pass one point during one second. The units for frequency are hertz . Traveling through the air, a music note with a frequency of 261.6 hertz (middle C) pushes air molecules back and forth 261.6 times every second. 

Frequency and wavelength are related to the amount of energy a wave has. For example, when making waves on a rope, it takes more energy to make a higher frequency wave. Moving your hand up and down 10 times per second (10 hertz) requires more energy than moving your hand only once per second (1 hertz). And those 10 hertz waves on the rope have a shorter wavelength than ones at 1 hertz. 

Many researchers rely on the properties and behavior of waves for their work. That includes astronomers, geologists and sound engineers. For example, scientists can use tools that capture reflected sound, light or radio waves to map places or objects. 

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The Speed of Light in Different Mediums: Exploring the Phenomenon

The speed of light is a fundamental constant in physics, but it can vary depending on the medium through which it travels. In a vacuum, light travels at a speed of approximately 299,792 kilometers per second (km/s). However, when light passes through different mediums, such as air, water, or glass, its speed can change. This change in speed is due to the interaction between light and the atoms or molecules in the medium. The speed of light in a medium is typically slower than its speed in a vacuum. To understand this variation , let’s take a look at the table below, which provides the speed of light in different mediums.

Key Takeaways

Understanding the speed of light, definition of the speed of light.

The speed of light is a fundamental concept in physics that refers to the speed at which light travels in a vacuum. In scientific terms , it is denoted by the symbol ‘c’. The speed of light is an essential parameter in various fields of study, including optics, electromagnetism, and relativity.

Light, which is composed of electromagnetic waves, can propagate through different mediums such as air, water, glass, and even diamond . However, the speed of light is medium-dependent, meaning it can vary depending on the optical properties of the material it travels through. This variation is due to the interaction of light with the atoms and molecules present in the medium.

The Constant Nature of Light Speed in Vacuum

In a vacuum, such as outer space , light travels at a constant speed of approximately 299,792,458 meters per second. This value is considered a fundamental constant of nature and is denoted by the symbol ‘c’. The speed of light in a vacuum is the maximum speed at which information or energy can be transmitted through space.

The constant nature of light speed in a vacuum has significant implications in various scientific phenomena . For instance, it plays a crucial role in the phenomenon of refraction, which occurs when light passes from one medium to another with a different optical density . According to Snell’s law, the velocity of light changes as it enters a different medium , resulting in a change in its direction .

The index of refraction is a measure of how much a particular medium can slow down the speed of light compared to its speed in a vacuum. It is defined as the ratio of the speed of light in a vacuum to the speed of light in the medium. Different materials have different index of refraction values, which determine how much they can bend or refract light.

Transparent materials , such as air, water, and glass, allow light to pass through them with minimal absorption or scattering. The speed of light in these materials is slower than in a vacuum due to the interaction between light and the atoms or molecules of the medium. The refractive index of these materials determines the extent of this slowdown .

In certain situations , when light encounters a boundary between two mediums , it can undergo total internal reflection. This phenomenon occurs when the angle of incidence is greater than the critical angle , which is determined by the refractive indices of the two materials . Total internal reflection is responsible for various optical phenomena , such as mirages and fiber optics communication .

The speed of light in different mediums is also related to the wavelength and frequency of light. In general, the speed of light decreases as the wavelength decreases or the frequency increases. This relationship is known as the dispersion of light and is responsible for the separation of colors in a prism .

The Speed of Light in Different Mediums

Light is a fascinating phenomenon that travels at incredible speeds through various mediums. The speed of light in different mediums can vary due to the properties of the material it is passing through. Let’s explore the speed of light in air, water, glass, and diamond.

Speed of Light in Air

In a vacuum or air, light travels at its maximum speed , which is approximately 299,792,458 meters per second. This speed is often denoted by the symbol ‘c’ and is a fundamental constant in physics. The speed of light in air is slightly lower than in a vacuum due to the presence of air molecules , but the difference is negligible for most practical purposes .

Speed of Light in Water

When light enters a medium such as water, its speed decreases. This decrease in speed is due to the optical density of water, which is a measure of how much the medium slows down the propagation of light. The speed of light in water is around 225,000,000 meters per second, which is approximately 75% of its speed in a vacuum.

Speed of Light in Glass

Glass is another transparent material that affects the speed of light. The speed of light in glass depends on the type of glass and its composition. On average, the speed of light in glass is around 200,000,000 meters per second, which is approximately 67% of its speed in a vacuum. The difference in speed is due to the higher optical density of glass compared to air or water.

Speed of Light in Diamond

Diamond is a unique material known for its exceptional optical properties . The speed of light in diamond is significantly lower compared to other mediums . It is approximately 124,000,000 meters per second, which is around 41% of the speed of light in a vacuum. This decrease in speed is due to the high refractive index of diamond, which measures how much the material bends light as it passes through.

The speed of light in different mediums is determined by the index of refraction, which is a measure of how much a medium slows down light compared to a vacuum. This phenomenon is governed by Snell’s law, which relates the angle of incidence and refraction of light as it passes from one medium to another. The velocity of light in a medium is inversely proportional to the index of refraction.

It’s fascinating to see how light behaves differently in various mediums. The speed of light in a medium is directly related to its optical density and index of refraction. Understanding these properties allows us to study light propagation, wavelength, frequency, dispersion, and even phenomena like critical angle and total internal reflection.

Factors Influencing the Speed of Light in Different Mediums

The role of material density.

The speed of light is a fundamental constant in physics, but it can vary when it travels through different mediums. One of the factors that influence the speed of light in different mediums is the material density .

Material density refers to the amount of mass per unit volume of a substance . In general, the denser the material , the slower the speed of light. This is because the density of a material affects how easily the atoms or molecules can interact with the electromagnetic waves of light. When light passes through a denser medium , such as water or glass, the atoms or molecules in the medium absorb and re-emit the light, causing a delay in its propagation .

The speed of light in a medium can be calculated using the formula:

where (v) is the velocity of light in the medium, (c) is the velocity of light in a vacuum, and (n) is the index of refraction of the medium. The index of refraction is a measure of how much the speed of light is reduced when it passes through a medium compared to its speed in a vacuum.

The Impact of Material Composition

Another factor that affects the speed of light in different mediums is the material composition . Different materials have different optical properties , which can influence how light interacts with them.

For example, transparent materials like air, water, glass, and diamond allow light to pass through them with varying degrees of ease. The atoms or molecules in these materials have specific arrangements that determine how they interact with electromagnetic waves. This interaction affects the speed at which light can propagate through the material.

The index of refraction, mentioned earlier, is a measure of how much a material slows down the speed of light. It is determined by the material’s composition and the arrangement of its atoms or molecules. Snell’s law, which relates the angles of incidence and refraction, can be used to calculate the index of refraction and determine how light bends when it enters a different medium .

The Effect of Temperature on Light Speed

Temperature is another factor that can influence the speed of light in different mediums. As the temperature of a medium changes , the speed of light passing through it can also change.

In general, as the temperature of a medium increases , the speed of light in that medium also increases. This is because an increase in temperature leads to an increase in the kinetic energy of the atoms or molecules in the medium. As a result , the atoms or molecules vibrate more vigorously, allowing them to interact more frequently with the electromagnetic waves of light and enabling the light to propagate faster.

On the other hand , at extremely low temperatures , such as near absolute zero, the speed of light can be significantly reduced. This is due to the decrease in the kinetic energy of the atoms or molecules, resulting in less frequent interactions with the light waves .

It is important to note that the effect of temperature on the speed of light in a medium is relatively small compared to other factors , such as material density and composition. However, it is still a factor to consider when studying the behavior of light in different mediums.

How the Speed of Light Changes in Different Mediums

The phenomenon of refraction.

When light travels from one medium to another, such as from air to water or from air to glass, it undergoes a phenomenon called refraction. Refraction is the bending of light as it passes through different mediums due to a change in its speed. This change in speed is a result of the interaction between light and the atoms or molecules of the medium it is passing through.

The speed of light in a vacuum is approximately 299,792,458 meters per second. However, when light enters a medium, its speed decreases. This decrease in speed is due to the interaction of light with the atoms or molecules of the medium. Different mediums have different optical densities , which determine how much they can slow down the speed of light.

The Concept of Optical Density

Optical density is a property of transparent materials that determines how much they can slow down the speed of light. It is related to the index of refraction, which is a measure of how much a medium can bend light. The index of refraction is defined as the ratio of the speed of light in a vacuum to the speed of light in the medium.

The formula for the index of refraction is:

Where: – (n) is the index of refraction – (c) is the speed of light in a vacuum – (v) is the speed of light in the medium

Different mediums have different index of refraction values, which means they can slow down light to different extents . For example, the index of refraction for air is approximately 1, while the index of refraction for water is around 1.33. This means that light travels faster in air compared to water.

The velocity of light in a medium is also dependent on the wavelength and frequency of the light. As light passes through a medium, its wavelength and frequency remain constant, but its velocity changes . This change in velocity can lead to interesting phenomena such as dispersion, where different wavelengths of light are bent at different angles .

In addition to air and water, other mediums like glass and diamond also have different index of refraction values. Glass has an index of refraction ranging from 1.5 to 1.9, depending on its composition, while diamond has an index of refraction of approximately 2.4. These variations in index of refraction allow for the manipulation of light in various optical devices .

Another important concept related to refraction is the critical angle . The critical angle is the angle of incidence at which light is refracted at an angle of 90 degrees with respect to the normal. When the angle of incidence exceeds the critical angle , total internal reflection occurs, where all the light is reflected back into the medium. This phenomenon is utilized in fiber optics , where light is transmitted through thin strands of glass or plastic using total internal reflection.

Calculating the Speed of Light in Different Mediums

Understanding the refractive index.

When light travels through different mediums, such as air, water, glass, or diamond, its speed can vary. This variation in speed is due to the interaction between light and the medium it is passing through. To understand this interaction , we need to explore the concept of the refractive index.

The refractive index, also known as the index of refraction, is a measure of how much a medium can slow down the velocity of light. It is denoted by the symbol ‘n’ and is defined as the ratio of the speed of light in a vacuum to the speed of light in the medium. Mathematically, it can be expressed as:

where ‘c’ represents the velocity of light in a vacuum and ‘v’ represents the velocity of light in the medium.

Different mediums have different refractive indices , which means they have varying abilities to slow down light. For example, the refractive index of air is approximately 1, while the refractive index of water is around 1.33. This means that light travels faster in air compared to water.

The Formula for Calculating Light Speed in a Medium

To calculate the speed of light in a medium, we can use the formula:

where ‘v’ represents the velocity of light in the medium, ‘c’ represents the velocity of light in a vacuum, and ‘n’ represents the refractive index of the medium.

By rearranging the formula, we can also calculate the refractive index of a medium if we know the velocity of light in that medium:

This formula allows us to determine the refractive index of different transparent materials , which is crucial for understanding how light propagates through them.

Practical Examples of Light Speed Calculation

Let’s consider a few practical examples to illustrate how to calculate the speed of light in different mediums.

• Air : The refractive index of air is approximately 1. Using the formula, we can calculate the speed of light in air as:

Therefore, the speed of light in air is the same as its velocity in a vacuum.

• Water : The refractive index of water is approximately 1.33. Using the formula, we can calculate the speed of light in water as:

This means that light travels at a slower speed in water compared to a vacuum or air.

• Glass : The refractive index of glass varies depending on its composition. For example, the refractive index of common window glass is around 1.5. Using the formula, we can calculate the speed of light in glass as:

Glass slows down the speed of light even more compared to water.

• Diamond : Diamond has a high refractive index of approximately 2.42. Using the formula, we can calculate the speed of light in diamond as:

Diamond significantly slows down the speed of light compared to other transparent materials .

These examples demonstrate how the refractive index of different mediums affects the speed of light. By understanding the refractive index and using the formula, we can calculate the speed of light in various mediums and gain insights into the behavior of light in different materials.

Measuring the Speed of Light in Different Materials

Light, as we know, is incredibly fast. But have you ever wondered how its speed varies when it travels through different materials? Measuring the speed of light in different mediums is a fascinating field of study that helps us understand the behavior of light and its interaction with various substances . In this article , we will explore both the traditional methods and modern techniques used to measure the speed of light in different materials.

Traditional Methods of Measuring Light Speed

Before the advent of modern technology , scientists devised ingenious methods to measure the speed of light in different mediums. One of the earliest methods involved observing the bending of light as it passed through transparent materials. This bending , known as refraction, occurs due to the change in the speed of light when it enters a medium with a different optical density .

To quantify this phenomenon , scientists introduced the concept of the index of refraction. The index of refraction is a dimensionless quantity that represents the ratio of the speed of light in a vacuum to its speed in a particular medium . It is denoted by the symbol ‘n’ and is calculated using Snell’s law, which relates the angles of incidence and refraction:

By measuring the angles of incidence and refraction, scientists could determine the index of refraction and subsequently calculate the speed of light in the medium using the formula:

where ‘v’ is the velocity of light in the medium and ‘c’ is the speed of light in a vacuum.

Modern Techniques for Light Speed Measurement

With advancements in technology, modern techniques for measuring the speed of light in different materials have emerged. These techniques rely on the principles of electromagnetic waves and the medium-dependent properties of light propagation.

One common method involves using a pulsed laser and a time-of-flight measurement system. By measuring the time it takes for a laser pulse to travel through a medium and bounce back, scientists can calculate the speed of light in that medium. This technique is particularly useful for measuring the speed of light in materials with high refractive indices , such as diamond.

Another modern technique utilizes the phenomenon of total internal reflection. When light travels from a medium with a higher refractive index to one with a lower refractive index , it can reach a critical angle beyond which it undergoes total internal reflection. By precisely measuring the critical angle , scientists can determine the refractive index and subsequently calculate the speed of light in the medium.

In addition to these techniques , researchers also employ advanced spectroscopic methods to study the dispersion of light in different materials. By analyzing the wavelength and frequency dependence of light as it passes through a medium, scientists can gain insights into the speed of light and its interaction with the material’s properties .

Why the Speed of Light is Different in Different Mediums

The theory of light propagation.

The speed of light is a fundamental constant in physics, denoted by the symbol ‘c’. In a vacuum, such as outer space , the speed of light is approximately 299,792,458 meters per second. However, when light travels through different mediums, such as air, water, glass, or diamond, its speed changes .

To understand why the speed of light varies in different mediums, we need to delve into the theory of light propagation. Light is an electromagnetic wave , consisting of oscillating electric and magnetic fields . These waves can travel through various materials , including transparent substances like air, water, and glass.

When light encounters a medium, it interacts with the atoms or molecules present in that material . This interaction causes the light to slow down as it traverses through the medium. The speed of light in a medium is determined by the optical density and the index of refraction of that medium.

The optical density of a medium is a measure of how much it slows down the speed of light compared to its speed in a vacuum. The index of refraction, denoted by ‘n’, quantifies this optical density . It is defined as the ratio of the speed of light in a vacuum to the speed of light in the medium.

The relationship between the velocity of light in a medium and the index of refraction can be expressed using Snell’s law:

Where ‘c’ is the speed of light in a vacuum and ‘v’ is the velocity of light in the medium.

The Influence of Medium on Light Speed

Different mediums have different optical densities and , consequently, different indices of refraction. This variation in optical density is what causes the speed of light to change when it travels through different materials.

Let’s take a look at the speed of light in some common transparent materials :

As we can see, the speed of light is fastest in a vacuum, where the index of refraction is 1. In air, the speed of light is only slightly slower due to its slightly higher index of refraction. However, when light enters denser mediums like water, glass, or diamond, its speed decreases significantly.

This change in speed is also accompanied by a change in the wavelength and frequency of the light. When light enters a medium with a higher index of refraction, its wavelength decreases, while its frequency remains constant. This phenomenon is known as dispersion.

Moreover, when light travels from a medium with a higher index of refraction to a medium with a lower index of refraction, it undergoes refraction. Refraction is the bending of light as it passes from one medium to another. The angle at which the light bends is determined by the difference in the indices of refraction between the two mediums and is governed by Snell’s law.

In certain cases , when the angle of incidence is large enough, light can experience total internal reflection. Total internal reflection occurs when light traveling in a medium with a higher index of refraction is incident on the boundary with a medium of lower index of refraction at an angle greater than the critical angle . This phenomenon is utilized in fiber optics and other optical devices .

Frequently Asked Questions

Q1: what is the speed of light in different mediums.

A1: The speed of light varies depending on the medium it travels through. It is fastest in a vacuum and slower in materials such as air, water, glass, and diamond.

Q2: How does the speed of light change in different materials?

A2: The speed of light in a medium is determined by its optical density , which is related to the index of refraction. As the optical density increases, the speed of light decreases.

Q3: Why is the speed of light different in different media?

A3: The speed of light is different in different media due to the interaction between electromagnetic waves and the atoms or molecules of the medium. This interaction affects the propagation of light.

Q4: How to calculate the speed of light in different mediums?

A4: The speed of light in a medium can be calculated using the formula v = c/n, where v is the velocity of light in the medium, c is the speed of light in a vacuum, and n is the refractive index of the medium.

Q5: Does the speed of light change in different materials?

A5: Yes, the speed of light changes in different materials. It is slower in materials with higher refractive indices and higher optical densities .

Q6: What determines the speed of light in a medium?

A6: The speed of light in a medium is determined by the medium’s optical density , which is related to its refractive index . The refractive index indicates how much the speed of light is reduced in the medium.

Q7: How does the speed of light vary in different mediums?

A7: The speed of light varies in different mediums based on their optical densities . Generally, denser mediums have higher refractive indices and slower speeds of light.

Q8: What happens to the speed of light as it travels in different mediums?

A8: As light travels from one medium to another, it can change its speed due to the interaction with the atoms or molecules of the new medium . This change in speed is known as refraction.

Q9: Why does the speed of light change in different mediums?

A9: The speed of light changes in different mediums because the electromagnetic waves interact with the atoms or molecules of the medium, causing a delay in their propagation .

Q10: What is the speed of light in another medium?

A10: The speed of light in another medium can be determined by knowing the refractive index of that medium and using the formula v = c/n, where v is the velocity of light in the medium and c is the speed of light in a vacuum.

• Polarized light microscopy
• Why do some surfaces reflect light while others absorb it
• Is it true that light can bend around corners
• Does the color of light affect its refractive index
• Why does light split into a spectrum when passing through a prism
• Is blue light from electronic screens harmful to our eyes
• Does a black hole bend light
• Energy and intensity of light waves
• Is it possible to slow down or stop light
• Refractive index calculations using speed of light

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All Our Senior SME are having more than 7 Years of experience in the respective fields . They are either Working Industry Professionals or assocaited With different Universities. Refer Our Authors Page to get to know About our Core SMEs.

Wave Behaviors

Light waves across the electromagnetic spectrum behave in similar ways. When a light wave encounters an object, they are either transmitted, reflected, absorbed, refracted, polarized, diffracted, or scattered depending on the composition of the object and the wavelength of the light.

Specialized instruments onboard NASA spacecraft and airplanes collect data on how electromagnetic waves behave when they interact with matter. These data can reveal the physical and chemical composition of matter.

Reflection is when incident light (incoming light) hits an object and bounces off. Very smooth surfaces such as mirrors reflect almost all incident light.

The color of an object is actually the wavelengths of the light reflected while all other wavelengths are absorbed. Color, in this case, refers to the different wavelengths of light in the visible light spectrum perceived by our eyes. The physical and chemical composition of matter determines which wavelength (or color) is reflected.

This reflective behavior of light is used by lasers onboard NASA's Lunar Reconnaissance Orbiter to map the surface of the Moon. The instrument measures the time it takes a laser pulse to hit the surface and return. The longer the response time, the farther away the surface and lower the elevation. A shorter response time means the surface is closer or higher in elevation. In this image of the Moon's southern hemisphere, low elevations are shown as purple and blue, and high elevations are shown in red and brown.

Absorption occurs when photons from incident light hit atoms and molecules and cause them to vibrate. The more an object's molecules move and vibrate, the hotter it becomes. This heat is then emitted from the object as thermal energy.

Some objects, such as darker colored objects, absorb more incident light energy than others. For example, black pavement absorbs most visible and UV energy and reflects very little, while a light-colored concrete sidewalk reflects more energy than it absorbs. Thus, the black pavement is hotter than the sidewalk on a hot summer day. Photons bounce around during this absorption process and lose bits of energy to numerous molecules along the way. This thermal energy then radiates in the form of longer wavelength infrared energy.

Thermal radiation from the energy-absorbing asphalt and roofs in a city can raise its surface temperature by as much as 10° Celsius. The Landsat 7 satellite image below shows the city of Atlanta as an island of heat compared to the surrounding area. Sometimes this warming of air above cities can influence weather, which is called the "urban heat island" effect.

Diffraction

Diffraction is the bending and spreading of waves around an obstacle. It is most pronounced when a light wave strikes an object with a size comparable to its own wavelength. An instrument called a spectrometer uses diffraction to separate light into a range of wavelengths—a spectrum. In the case of visible light, the separation of wavelengths through diffraction results in a rainbow.

A spectrometer uses diffraction (and the subsequent interference) of light from slits or gratings to separate wavelengths. Faint peaks of energy at specific wavelengths can then be detected and recorded. A graph of these data is called a spectral signature. Patterns in a spectral signature help scientists identify the physical condition and composition of stellar and interstellar matter.

The graph below from the SPIRE infrared spectrometer onboard the ESA (European Space Agency) Herschel space telescope reveals strong emission lines from carbon monoxide (CO), atomic carbon, and ionized nitrogen in Galaxy M82.

Scattering occurs when light bounces off an object in a variety of directions. The amount of scattering that takes place depends on the wavelength of the light and the size and structure of the object.

The sky appears blue because of this scattering behavior. Light at shorter wavelengths—blue and violet—is scattered by nitrogen and oxygen as it passes through the atmosphere. Longer wavelengths of light—red and yellow—transmit through the atmosphere. This scattering of light at shorter wavelengths illuminates the skies with light from the blue and violet end of the visible spectrum. Even though violet is scattered more than blue, the sky looks blue to us because our eyes are more sensitive to blue light.

Aerosols in the atmosphere can also scatter light. NASA's Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite can observe the scattering of laser pulses to "see" the distributions of aerosols from sources such as dust storms and forest fires. The image below shows a volcanic ash cloud drifting over Europe from an eruption of Iceland's Eyjafjallajökull Volcano in 2010.

Refraction is when light waves change direction as they pass from one medium to another. Light travels slower in air than in a vacuum, and even slower in water. As light travels into a different medium, the change in speed bends the light. Different wavelengths of light are slowed at different rates, which causes them to bend at different angles.

Next: Visualization: From Energy to Image

National Aeronautics and Space Administration, Science Mission Directorate. (2010). Wave Behaviors. Retrieved [insert date - e.g. August 10, 2016] , from NASA Science website: http://science.nasa.gov/ems/03_behaviors

Science Mission Directorate. "Wave Behaviors" NASA Science . 2010. National Aeronautics and Space Administration. [insert date - e.g. 10 Aug. 2016] http://science.nasa.gov/ems/03_behaviors

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Does Light Travel Forever?

Most recent answer: 01/23/2013

Hi Raja, Good question. First, let's think about why sound does not travel forever. Sound cannot travel through empty space; it is carried by vibrations in a material, or medium (like air, steel, water, wood, etc). As the particles in the medium vibrate, energy is lost to heat, viscous processes, and molecular motion. So, the sound wave gets smaller and smaller until it disappears. In contrast, light waves can travel through a vacuum, and do not require a medium. In empty space, the wave does not dissipate (grow smaller) no matter how far it travels, because the wave is not interacting with anything else. This is why light from distant stars can travel through space for billions of light-years and still reach us on earth. However, light can also travel within some materials, like glass and water. In this case, some light is absorbed and lost as heat, just like sound. So, underwater, or in our atmosphere, light will only travel some finite range (which is different depending on the properties of the material it travels through). There is one more aspect of wave travel to consider, which applies to both sound and light waves. As a wave travels from a source, it propagates outward in all directions. Therefore, it fills a space given approximately by the surface area of a sphere. This area increases by the square of the distance R from the source; since the wave fills up all this space, its intensity decreases by R squared. This effect just means that the light/sound source will appear dimmer if we are farther away from it, since we don't collect all the light it emits. For example, light from a distant star travels outward in a giant sphere. Only one tiny patch of this sphere of light actually hits our eyes, which is why stars don't blind us! David Schmid

(published on 01/23/2013)

Follow-Up #1: How far does light go?

Light just keeps going and going until it bumps into something.  Then it can either be reflected or absorbed.  Astronomers have detected some light that has been traveling for more that 12 billion years, close to the age of the universe.   

Light has some interesting properties.   It comes in lumps called photons.  These photons carry energy and momentum in specific amounts related to the color of the light.  There is much to learned about light.   I suggest you log in to our website and type  LIGHT into the search box.   Lots of interesting stuff there.

To answer your previous question "Can light go into a black hole?" ,  the answer is yes.

(published on 12/03/2015)

Follow-Up #2: less than one photon?

Certainly you can run the ouput of a single-photon source through a half-silvered mirror, and get a sort of half-ghost of the photon in two places. If you put ordinary photon detectors in those places, however, each will either detect zero or one. For each source photon, you'll get at most one of the detectors to find it. How does the half-ghost at the other one know whether it's detectably there or not? The name of that mystery is "quantum entanglement". At some level we don't really know the answer.

(published on 02/04/2016)

Follow-Up #3: stars too far away to see?

Most stars are too far for us to see them as individual stars even with our best telescopes. Still, we can get light from them, mixed with light from other stars. If our understanding of the universe is at all right, there are also stars that once were visible from here but now are outside our horizon so no light from them reaches us. It's probable that there are many more stars outside our horizon than inside, maybe infinitely more. It's hard to check, however, what's happening outside our horizon! It's even hard to define what we mean by "now" for things outside the horizon.

(published on 07/22/2016)

Follow-Up #4: light going out to space

Certainly ordinary light travels out to space. That's how spy cameras and such can take pictures of things here on the Earth's surface.

(published on 09/01/2016)

Follow-Up #5: end of the universe?

We don't think there's any "end" in the sense of some spatial boundary. Unless something changes drastically, there also won't be an end in time. The expansion looks like it will go on forever. So that wouldn't give a maximum range.

(published on 03/26/2017)

Follow-Up #6: seeing black holes

In principle a well-aimed beam would loop around the outside of the black hole and return to Earth. There aren't any black holes close enough to make this practical. Instead the bending of light by black holes is observed by their lensing effect on light coming from more distant objects.

The amazing gravitational wave signals observed from merging black holes provide even more direct and convincing proof that black holes exist and follow the laws of General Relativity.

(published on 01/29/2018)

Follow-up on this answer

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Paul Krugman

Ukraine aid in the light of history.

By Paul Krugman

Opinion Columnist

On Saturday the House of Representatives finally overcame MAGA opposition and approved a new aid package for Ukraine. The Biden administration presumably had matériel ready to ship , just waiting for congressional authorization, so the effects of this legislative breakthrough will be quick.

Like many observers, I’m simultaneously relieved, ashamed, angry and worried by what has happened. I’m relieved that a nation under siege will probably — probably — get aid in time to survive, at least for a while, something that was increasingly in doubt given overwhelming Russian artillery superiority. I’m ashamed that things got to this point — that America came so close to betraying a democracy in danger. I’m angry at the political faction that blocked aid for so many months, not, as I’ll explain below, because of reasonable concerns about the cost, but probably because they want Vladimir Putin to win. And I’m worried because that faction remains powerful — a majority of Republicans in the House voted against Ukraine aid — and could still doom Ukraine in the years ahead.

But let me set emotions aside and try to do some analysis. In particular, let me take on some myths about aid to Ukraine. No, spending on Ukraine isn’t a huge burden on America, coming at the expense of domestic priorities. No, America isn’t bearing this cost alone, without help from our European allies. Yes, U.S. aid is still crucial, in part because Europe can supply money but isn’t yet in a position to supply enough military hardware.

To understand these points, I find it useful to look back at the obvious historical parallel to current aid to Ukraine: Franklin Roosevelt’s Lend-Lease program, which began delivering aid to Britain and China in 1941, before Pearl Harbor brought America officially into World War II.

It is often forgotten how controversial that aid was at the time. Many people are probably aware that there was an America First movement that opposed any aid to embattled Britain, in part because some of its prominent leaders, notably Charles Lindbergh , were racist and openly sympathetic to the Nazis.

I suspect that fewer people are aware that even in Congress, Lend-Lease was a deeply partisan issue. The initial bill, enacted in early 1941, passed the House with very little Republican support. Even more strikingly, support for Lend-Lease (triangles pointing up in the chart below) was closely correlated with economic ideology (Dimension 1). Almost all liberals favored supporting Britain in its darkest hour; many conservatives didn’t:

Yet the aid passed. Congress appropriated $13 billion before the attack on Pearl Harbor. This was an immense sum at the time — about 10 percent of America’s annual gross domestic product . Somewhat surprisingly, however, not much of that total consisted of weaponry. As the American Historical Association noted: “Our munitions industry was still largely in the tooling up state. And the flow of finished weapons was at first only a trickle.”

Indeed. Europe had begun rearming years before World War II started, while an isolationist United States hadn’t developed much of a defense industry — to take a famous example, the Sherman tank didn’t go into production until 1942. As a result, most of America’s initial aid took the form of food — at first we were less the arsenal of democracy than its breadbasket.

How does aid to Ukraine compare with that experience?

First, it’s vastly smaller relative to the size of our economy. The just-passed package will roughly double the cumulative aid we’ve given Ukraine, but at about $60 billion it’s less than one-fourth of 1 percent of G.D.P. — around one-fortieth the size of the initial Lend-Lease appropriation. Anyone claiming that spending on this scale will break the budget, or that it will seriously interfere with other priorities, is innumerate, disingenuous or both.

What about claims that America is bearing too much of the burden? Last week Donald Trump accused Europe of failing to pay its share : “Why is it that the United States is over $100 Billion Dollars into the Ukraine War more than Europe, and we have an Ocean between us in separation? Why can’t Europe equalize or match the money put in by the United States of America in order to help a Country in desperate need?” (Eccentric, more or less Germanic capitalization in the actual post.)

The answer to his questions is that his assertions are false. As the Kiel Institute reports, “The data show that total European aid has long overtaken U.S. aid — not only in terms of commitments, but also in terms of specific aid allocations sent to Ukraine.” Notably, many though not all European nations are spending substantially more in support of Ukraine as a percentage of G.D.P. than we are:

What is true is that the United States has provided more military aid than Europe:

Why? Remember that in the first year of Lend-Lease, America couldn’t supply much in the way of weapons, despite the immense size of our economy, because years of low military spending had left us with an underdeveloped military-industrial base. It took a couple of years to translate America’s overall industrial might into comparable military might. Right now Europe is in a similar situation: It has the money to help Ukraine, and for the most part it has the will, but it doesn’t have the production capacity to meet Ukraine’s military needs.

Will this change? Europe is moving toward increased military capacity , but more slowly than it should, and American aid remains essential.

So as I said, I’m relieved that America has finally released essential aid, but still very worried about the future. For now, at least, U.S. support remains crucial to Ukraine’s survival.

On being wrong about military spending .

A senior Republican speaks about the G.O.P. and Ukraine.

Have Democrats de facto taken control of the House?

Meanwhile, the U.S. economy keeps powering ahead .

Facing the Music

Just finished a bike trip in Sicily. A lot of the best architecture is from the 18th century, when Italy was still more of a cultural powerhouse than I had realized.

Paul Krugman has been an Opinion columnist since 2000 and is also a distinguished professor at the City University of New York Graduate Center. He won the 2008 Nobel Memorial Prize in Economic Sciences for his work on international trade and economic geography. @ PaulKrugman

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• Traffic Lab

Your how-to guide for the new Eastside light rail line

The East Link Starter Line, also called the 2 Line, begins service 11 a.m. Saturday with eight stops in Bellevue and Redmond. If you’re a newcomer to Sound Transit light rail, here’s what to know.

Hours: Eastside trains are scheduled to arrive every 10 minutes, seven days a week from 5:30 a.m. to 9:30 p.m.

Travel time: A ride on the 6-mile line, end to end, lasts 20 minutes.

Capacity: Two-car trains are intended to carry 300 people, when half are seated and half standing.

Locations: At the south end is South Bellevue Station. From there, trains head northeast to East Main, Bellevue Downtown, Wilburton, Spring District, BelRed, Overlake Village and the last stop, Redmond Technology Station. Free park-and-ride space is available at South Bellevue (1,500 stalls), BelRed (300), Overlake Village (203 stalls, four blocks away) and Redmond Technology (300). The other four stations include passenger drop-off sites and connecting bus stops.

Fares: Standard adult fares are $2.25 to $2.50 depending on distance. Link light rail charges $1 for 65+, disabled and low-income passengers holding a discounted ORCA fare card. People 18 and younger ride transit free throughout Washington state. Fares will be collected opening weekend.

How to pay: Fares are paid before entering trains. Most people use a regional ORCA fare card, tapping it on a yellow detector near the station entrance. Tap again when leaving a station to avoid overcharges. See orcacard.com to order fare cards online or find in-person sites . ORCA cards are available at many QFC and Fred Meyer stores. Paper single-trip and all-day tickets are sold in station vending machines, which accept cash or credit/debit cards. If you’re taking more than a couple of rides, it’s simpler to buy an ORCA fare card directly from the ticket machine. You pay $3 for the card, then load it with funds. Or download the Transit Go app .

Station entry: There are no turnstiles. “Fare ambassadors” canvass some trains to check for proof of payment and help people navigate. They issue advice or warnings and can cite repeat evaders for $50 or more. Pedestrians should look all ways for trains next to station platforms and at grade crossings around BelRed Station.

Bicycles: Bikes are allowed onboard light rail, hanging from a hook in a nook that doubles as luggage space.

Restrooms: There are no public restrooms in the eight stations.

Buses: The primary train-bus connections are I-90 routes that swing by South Bellevue Station; Highway 520 routes next to Redmond Technology Station; and I-405 buses that converge at Bellevue Downtown Station. Metro’s B Line bus links the Crossroads neighborhood to multiple train stops including Wilburton.

Traffic Lab | Eastside Light Rail

• The highs and lows of Eastside’s 8 new light rail stations, from parking to traffic hazards
• New to light rail? Here’s your starter kit for riding
• Eastside’s light rail wait is almost over with ‘Starter Line’
• Map: Eastside’s first light rail stations open April 27
• Timeline: Eastside light rail has been a long time coming

Traffic Lab reporter Mike Lindblom is taking questions for an upcoming story. Email him at [email protected]

Most Read Local Stories

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• Woodland Park Zoo's 45-year-old hippo, Water Lily, to be euthanized
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• What to expect when you try the Eastside's eight light rail stations

The opinions expressed in reader comments are those of the author only and do not reflect the opinions of The Seattle Times.

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Airlines will now be required to give automatic cash refunds for canceled and delayed flights

FILE - Passenger drop off their baggage at United Airlines in C Terminal at George Bush Intercontinental Airport, Thursday, Dec. 21, 2023, in Houston. The Biden administration issued final rules Wednesday, April 24, 2024, to require airlines to automatically issue cash refunds for things like delayed flights and to better disclose fees for baggage or canceling a reservation. (Brett Coomer/Houston Chronicle via AP, File)

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The Biden administration issued final rules Wednesday to require airlines to automatically issue cash refunds for things like delayed flights and to better disclose fees for baggage or canceling a reservation.

The Transportation Department said airlines will be required to provide automatic cash refunds within a few days for canceled flights and “significant” delays.

Under current regulations, airlines decide how long a delay must last before triggering refunds. The administration is removing that wiggle room by defining a significant delay as lasting at least three hours for domestic flights and six hours for international ones.

Airlines still will be allowed to offer another flight or a travel credit instead, but consumers can reject the offer.

The rule will also apply to refunds of checked-bag fees if the bag isn’t delivered within 12 hours for domestic flights or 15 to 30 hours for international flights. And it will apply to fees for things such as seat selection or an internet connection if the airline fails to provide the service.

Complaints about refunds skyrocketed during the COVID-19 pandemic, as airlines canceled flights and, even when they didn’t, many people didn’t feel safe sharing a plane cabin with other passengers.

Airlines for America, a trade group for large U.S. carriers, noted that refund complaints to the Transportation Department have fallen sharply since mid-2020. A spokesperson for the group said airlines “offer a range of options — including fully refundable fares — to increase accessibility to air travel and to help customers make ticket selections that best fit their needs.”

The group said the 11 largest U.S. airlines issued $43 billion in customer refunds from 2020 through 2023.

The Transportation Department issued a separate rule requiring airlines and ticket agents to disclose upfront what they charge for checked and carry-on bags and canceling or changing a reservation. On airline websites, the fees must be shown the first time customers see a price and schedule.

The rule will also oblige airlines to tell passengers they have a guaranteed seat they are not required to pay extra for, although it does not bar airlines from charging people to choose specific seats. Many airlines now charge extra for certain spots, including exit-row seats and those near the front of the cabin.

The agency said the rule will save consumers more than $500 million a year.

Airlines for America said its members “offer transparency and vast choice to consumers” from their first search.

The new rules will take effect over the next two years. They are part of a broad administration attack on what President Joe Biden calls “junk fees.” Last week, Transportation Secretary Pete Buttigieg announced that his department will let state officials in 15 states help enforce federal airline consumer protection laws .

Travel firm Viking aims for up to $10.8 bln valuation in US IPO

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Reporting by Jaiveer Singh Shekhawat in Bengaluru; Editing by Sonia Cheema and Maju Samuel

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Gulf bourses end mixed; Egypt extends decline

Stock markets in the Gulf put in a mixed performance on Sunday amid rising oil prices and receding hopes of rapid interest rate cuts by the U.S. Federal Reserve after a series of inflation readings.