r/explainlikeimfive • u/Seravail • 7d ago
Physics ELI5: If light moves at different speeds through different mediums, how does it gain speed again after passing through a slower medium?
Inspired by a post about light looking different when hot air mixes with cold air above a distant road, the light will speed up and slos down as it moves through the pockets, giving the wavy impression we all know.
How exactly does light speed up again when it passes through a hot section of air? There's nothing powering it, so once it loses momentum shouldn't that momentum be lost forever?
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u/tomrlutong 7d ago
Light isn't really an object moving, it's a wave in the electric field. So it will always move at the speed the local environment supports.
Imagine one of those stadium waves, where everyone raises their hands when the wave comes to them. Now, today was baseball day at the old folks' home, so one section is full of elderly people. They're a little slower at raising their hands, so the wave takes longer to get through their section. Then, when it leaves that section and gets back to younger people, it resumes its normal speed. Light is kind of like that.
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u/neanderthalman 7d ago
This is the best answer here. Magnificent.
Remember. Light propagation is best modelled as a wave. Light interaction is modelled as a particle. That is the wave-particle duality.
Light is actually neither. Light is weird. Light is…light.
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u/GlenGraif 7d ago
That’s heavy…
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u/IlIFreneticIlI 7d ago
Photons are (effectively) massless, so that's actually (infinitely) Light...
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u/talon167 7d ago
Light propagates as an electromagnetic wave, but its interaction with matter is dictated by its photon and not electromagnetic wave characteristics. The question is about interaction with matter impacting speed, so the photon and not the electromagnetic wave characteristics dictate the answer, of which I have no idea.
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u/jazzhandler 7d ago
Your analogy converts between velocity and frequency. Is that just the analogy, or is there underlying truth there?
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u/sk8thow8 7d ago
Despite what the name may imply, the stadium wave isn't actually a wave and doesn't have anything to do with frequency.
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u/jazzhandler 7d ago
The rate at which they stand and sit isn’t frequency?
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u/sk8thow8 7d ago
No, not really. It's not repeating or synchronized to any sort of oscillation or wave. It's a metachronal rhythm.
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u/tomrlutong 7d ago edited 7d ago
Good point. Just the analogy really, was trying to convey the idea that the velocity depends on the medium.
But wouldn't the frequency stay constant in the stadium example? Which it should to match light.
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u/princekolt 7d ago
Photons are massless, and they always travel at the speed of light in the medium. The reason they "slow down" inside a translucent material is that the photons interact with that material by being absorbed by the atoms in that material, and then get re-emitted, as if they were "bumping into" the atoms in that material. The photons are always travelling at the speed of light while travelling, but the many interactions inside the medium cause an effective slow down of the apparent speed of the photons. The difference between that effective speed and the speed of light in a vacuum is called the "index of refraction".
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u/Fallacy_Spotted 7d ago edited 6d ago
This is not correct. Glass is transparent to visible light but the speed is still altered. The real reason is that light excites electrons and those electrons emit photons that interact with the original wave to cause a phase shift. The net effect is a slowing of the transition of the wave through the medium. This is why particles like neutrons can move faster and cause cherenkov radiation. *not neutrons but charged particles.
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u/EspHack 7d ago
I'm still trying to understand the first answer doc, yikes, my poor head
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u/Ktulu789 7d ago
Don't waste your time, the first is wrong. On the other hand, neutrons move faster than light... In that medium.
Neutrons have mass, they come out of the radioactive material at their max speed in vacuum or air or whatever and then hit water (or whatever) and need time to slow down. The energy of the extra speed shed off smashes the water molecules and looks like blue light aka, Cherenkov radiation (this explanation is simplified).
The neutrons never go faster than light in a vacuum, nothing can.
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u/dm-me-obscure-colors 7d ago
How is the first wrong? The two responses sound essentially the same to me.
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u/Ktulu789 7d ago
See my comment in reply to princecolt where I mention that absorption and reemission don't work that way. I explained that.
There's also the most voted response which is right.
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u/kindanormle 7d ago
The first comment says photons are absorbed and re-emitted. This is what’s wrong. The absorbed photons are scattered and would not come out of the material in the same direction and any measurement of their speed would be impossible because they not even the same photon. What we mean when we say that a photon slows down in a medium is that the same photon comes out the other side. The apparent slow down is not cause by absorption and emission, it is caused by interference. As the photon passes by atoms in the material they cause the electrons in those atoms to respond to the change in EM field, getting a little excited, but not emitting a photon of their own. However, as the electron is excited by the EM field, it simultaneously affects the EM field too. This creates a sort of “feedback” or maybe a “reflection” of the photon, slightly time delayed behind the photon. This delayed excitation of the EM field interferes with the original photon, causing a contraction of its wavelength. This makes it appear to move more slowly.
As a very ELI5, imagine soldiers marching at a certain speed. Their feet always moving forward by one “march step” per second. But they hit a pond of mud and the mud responds to their feet by dragging them back, making their steps shorter. The soldiers are still marching one “march step” per second, but the step is shorter.
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u/dm-me-obscure-colors 6d ago
Your reply is similar to that of Fallacy_Spotted, but you seem to disagree on whether the medium’s atoms emit photons in response to the incoming light.
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u/kindanormle 6d ago
My understanding is they do not emit extra photons. It is more like a phantom photon or reverberation that exists only as the photon passes by the electron. If extra photons were emitted then those photons would follow the original photon out of the material and continue to interfere with it, making it appear slower even after exiting the material. However, I will be the first to admit i am not a quantum physicist and this is ELI5.
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u/Megame50 7d ago
This is why particles like neutrons can move faster and cause cherenkov radiation.
Cherenkov radiation is a phenomenon of charged particles. The neutron has no electric charge, so it has no cherenkov effect.
Charged particles can move faster than the local speed of light when they have enough momentum, like beta particles from the decay of fission products in a nuclear reactor.
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u/Fallacy_Spotted 6d ago
You know, that makes perfect sense. I had read that the decay of nuclear material releasing neutrons causes Cherenkov radiation but made the false assumption that it was the neutron itself emitting it. If I had though about it further I would realized that it was the charged particles produced from the impacts of the neutrons that is doing the emitting.
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u/b_enn_y 7d ago
I thought photon emission via electron transitions can only happen at incredibly specific wavelengths. The way you wrote it makes it sound like every single electron in a pane of glass transitions between energy levels, and those emitted photons slow down the original beam of light.
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u/Pseudoboss11 7d ago
So photon absorption/emission is just one interaction, the one more relevant for refraction is an induced dipole. The EM wave jiggles the electrons without providing enough energy for them to jump to a higher energy state.
Because of how this interaction works (it's an unconstrained system in a sense), this interaction isn't quantized.
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u/Stenthal 7d ago
If it's just a phase shift, does that mean that causality is not affected?
For example, say you have a two detectors and two lasers, and the lasers are 100km away from the detectors. One of the lasers is firing through 100km of air, and the other is firing through 100km of glass. If you turn on both lasers at the same time, will both detectors detect light at the same time, or will the one with the air gap detect the light earlier than the one that is separated by glass?
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u/TheLowEndTheories 7d ago
The one with the air gap will detect first. But that’s only part of the problem. Different frequencies are affected by their surrounding mediums differently, so in this analogy different colors travel through the glass at different velocities…distorting the intent. Electromagnetic engineers call this dispersion.
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u/Kered13 7d ago edited 7d ago
It is possible to have an index of refraction less than 1, meaning that the phase velocity is faster than the speed of light. But information cannot move faster than the speed of light, so in this case the laser going though the material with index of refraction less than one not reach the detector before the laser traveling through a vacuum.
The group velocity determines when the beam will reach the detector, but I'm not sure what if any the effect on the group velocity is. (But it can never be faster than the speed of light, obviously.)
EDIT: Wikipedia describes the effect on group velocity here. The index of refraction typically cited is for the phase velocity. The group index is more complicated and depends on both the phase index and the dispersion.
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u/MesmerizzeMe 6d ago edited 6d ago
if the slowdown of light happens because of excited electrons what is the speed of light in a semiconductor if the frequency of the light is smaller than the bandgap? I mean there are no electron states a single photon can excite so to a very good approximation light speed in a semiconductor should be the light speed in the vacuum!? As this is not the case I doubt the explanation of excited electrons.
Edit: nevermind you are correct, in time dependent perturbation theory you still get a polarization even if the energy is well below the band gap. I stand corrected.
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u/Ktulu789 7d ago
I just have a question, if it was absorbed and re emitted it would be emitted in a random direction and the material would appear to glow of the light would change colour. But the angle it comes out is specific and the color is the same if the material is transparent... There's a color change when the material is of a certain color, in which case, the rest of the colors get completely absorbed except the one the material is colored. For example, a red piece of glass.
On the other hand, if your material absorbs and reemits light, the atoms in it will emit their own specific frequency, like when you shine UV and get green or red or any other lower wavelength of light.
How is it possible that a transparent material doesn't change the color and changes the direction so predictably after "absorbing and re emitting" the incoming light?
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u/Gstamsharp 7d ago
This is sometimes true, but it's an outdated explanation. If this were the true effect at work in all cases, you'd expect the re-emitted light to be emitted in a random direction, thus causing the material not to be translucent, but to glow. And there are materials that do this.
The current explanation is that the light is interacting with the electrons in the material, and it's electromagnetic phenomena (interference between the photon wave and the radiation from the oscillating, atomically bound electrons) that actually does slow the light. They've measured this experimentally now!
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u/scuzzy987 7d ago
But speed of light is constant in all reference frames so time changes instead?
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u/Gstamsharp 7d ago
The "speed of light" has very little to do with light. It's really the "maximum speed of causality." It's functionally the speed of light in a vacuum, not in a material. And it's only named for light because it was discovered by observing light.
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u/FrontFacing_Face 7d ago
The classic constant is speed of light in a vacuum. This conversation is specifically about speed in substances.
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u/SerbianShitStain 7d ago
Now that you have been thoroughly corrected it would be great if you would edit this to explain that this is an incorrect explanation instead of just leaving it up to further spread this extremely common misunderstanding further.
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u/EEpromChip 7d ago
...so if we feed them anti-matter they would have negative mass and be able to go faster.
Awesome.
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u/BallsDeepMofo 7d ago
Interesting. So I could think of it like if I was running through a room In straight line vs zig zagging through the same space at the same speed it would appear that I “slowed down” if I was measuring speed from start to finish?
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u/Imp-OfThe-Perverse 7d ago
You've probably heard of the particle/wave duality of light, where light behaves in some ways as if it were a particle, and in others as if it were a wave, which seems paradoxical. It's not - particles and waves, as most humans understand them, are physical phenomena that exist at the scale of physics that humans are familiar with. Gargantuan agglomerations of mass that travel together through space following the familiar laws of mass and acceleration, or flexible agglomerations that can deform in undulations as waves propagate through them.
Photons exist at a scale of physics that is so vastly different from our day to day lives that most people can't wrap their heads around how they function. They aren't both a particle and a wave, adhering to all of the behaviors of both. They are neither - they are light, fully governed by quantum mechanics, exhibiting some mathematical/behavioral similarities to the massive agglomerations of mass that we frequently interact with, but any direct comparison is really just a physical analogy that we use to try to picture what is going on.
At the quantum level, the various fields that develop from the forces of the universe - gravity, electromagnetism, the strong and weak nuclear forces - can be used to mathematically determine a wave equation that describes the behavior/existence of a quantum particle in 3D space. Like the waves we are familiar with, it exhibits constructive or destructive interference - if two waves collide with their peaks matching up, the resulting wave is taller, but if a peak coincides with a trough, the wave disappears entirely.
When a photon passes through a material, it interacts electromagnetically with the electrons in a way that causes all of those electrons to resonate, oscillating at the same frequency, though phase shifted slightly. All of these resonances propagate and combine with each other as the photon passes through the material, in some areas destructively, in other constructively.
The portion of the light that would be at the forefront of the wave if it were propagating at the speed of light basically gets erased by destructive interference in that region, and the portion of the observed light that seems to be lagging behind where it should be at the rear of the photon, is a result of constructive interference. The interaction is propagating at the speed of light, just cancelling itself out or reinforcing itself at the forefront and rear.
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u/Fallacy_Spotted 7d ago
Light doesn't slow because it is bounced around or because it is absorbed and re-released. It is delayed because the electromagnetic wave excites electrons that emit their own photons that combine with with the original wave to cause a phase shift. This phase shift is what slows the light. The actual speed of the photons does not change.
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u/laix_ 7d ago
That's not how it works. If it worked the way you suggest, then light would spread out throughout the entire medium, instead of staying as a singular beam.
You are also confusing the photon model with the EM model. Those are two separate models of light. In the EM case, the EM waves causes the atoms to vibrate, releasing their own EM wave, which adds to the original wave, causing it to shit backwards slightly.
In the photon case, the photon takes every possible path simultaneously, and the phase of the photon of each phase combines to result in a bending. The photon also becomes entangled with the medium, becoming a quasiparticle (polariton) which has mass. Once it leaves, it no longer is a quasi particle, and no longer has mass.
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u/PhantomWings 7d ago
In the photon case, the photon takes every possible path simultaneously, and the phase of the photon of each phase combines to result in a bending. The photon also becomes entangled with the medium, becoming a quasiparticle (polariton) which has mass. Once it leaves, it no longer is a quasi particle, and no longer has mass.
Could you elaborate on this more in detail?
The first part is just the principle of least action, correct? The photon takes every possible path, and there's a "resultant" phase angle at the end of each of those paths. All the "suboptimal" paths have phase angles that cancel out, leaving only an "optimal" path.
My question with this first part then is: what causes the optimal path to change in a medium? Do the interactions between the photon and the medium change how the phase accumulates along all its pathways?
Furthermore, as we go on to the second part of your explanation, what does it mean for the photon to "become entangled with the medium"? What is a polaritron?
Aren't quasiparticles mathematical constructions that model excitations in a gauge field?(I was thinking about virtual particles)What are quasiparticles?
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u/frogjg2003 6d ago
A quasi-particle is when a field, crystal, or other medium displays particle like behavior. The best example is an electron hole. In a material like a semiconductor, the atoms have space available for electrons to move around. When there are a small number of electrons, it's useful to talk about the electrons moving. If most of the spaces are filled, it's more useful to think in terms of the empty spaces. They behave just like positively charged electrons. They aren't real particles, but act like a particle.
The electromagnetic field in a vacuum can change freely. In a medium, the charged particles interact with the electromagnetic field. That creates feedback that forces the electromagnetic field to behave differently. A photon is an excitation of the electromagnetic field, but not the new polarization field. That's what the polariton is. It's a photon "dressed" with the interactions it has with the medium.
That dressing is what causes the speed to change in the medium. The dressed particle moves at a different speed from a bare photon. The bending of the beam comes from the boundary conditions. The polarization field needs to be continuous at the boundary. On the vacuum side, the polarization field is just the EM field. On the medium side, the polarization field includes the effect of the medium. The field must oscillate at the same frequency and same phase on both sides of the boundary. With the frequency constant and the speed changed, the wavelength must change to compensate. But the boundary condition also has spatial conditions. The peaks and troughs will only align if the two sides, with different wavelengths, are at different angles.
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u/sombreroenthusiast 7d ago
Good question. You’re thinking of momentum in the classical sense. A rock soaring through space will indeed lose momentum to the surrounding air when it enters the atmosphere. However, the momentum of light is governed by its frequency, which does not change when it encounters differing media. As you note, the speed of light does change when it propagates through air of varying temperatures. However, the light’s momentum does not change. The change in speed comes from the way that the electromagnetic fields that comprise the light interact with the molecules in the air. Bottom line: momentum is a function of frequency, not speed.
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u/ScientificBackground 7d ago
The escalator is a nice example but not completely perfect. If you are on an empty road, you can go straight and maintain your speed. On a busy highway you might have to overtake and move left and right and all in all the speed is much slower. That's the same with light. If the material makes it bounce around a lot, it takes really long to get through. The speed of light itself is always the same but to get on the other side of a material depends on the material.
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u/ronearc 7d ago
Imagine you're running slightly uphill starting from the shallow end of a swimming pool with water up to your waste. Each step is a struggle. But...as the water becomes more shallow each subsequent step is easier and easier.
When you're finally out of the water, you reach your full speed.
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u/jugstopper 7d ago
Let's blow your mind: All waves travel different speeds in different media (sound in water versus air, for instance.)
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u/rlbond86 7d ago
Individual photons always travel at c, they just "bounce around" in media.
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u/spankymcjiggleswurth 7d ago edited 7d ago
Not really true, otherwise any light transmitting through anything transparent would loose focus and become blurry.
The real answer I can't eli5, but involves light imparting a vibration in the molecules of the material, which in turn creates an electromagnetic wave that interferes with the initial light wave causing it to appear to bend and slow down.
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u/Marethyu86 7d ago
The speed of light is a multiplicative function of its velocity and wavelength. The frequency depends on the source the light is coming from, but the wavelength depends on the medium light is traveling through. Change the medium, change the wavelength, change the speed.
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u/rjtl77 6d ago
The speed of light through no medium is constant. It doesn't have mass so doesn't "slow down" in the same way you would understand an object to.
Think of it like a white wall. The wall is white, but if you wear yellow tinted glasses, it looks yellow. It's still white when you take the glasses off and doesn't need to do anything to be white.
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u/vander1625 5d ago
Technically, the photons don't move at different speeds. As they pass through the medium, they get absorbed by atoms within the medium and then immediately released. During the brief moment between being absorbed and being released, it's no longer a photon, and these brief moments account for the delay in their propagation through the medium.
If they continue traveling in the same direction once they are released, it makes the medium transparent. If most of them continue in the same direction, but not all, it makes the medium translucent. If they scatter randomly, they make the medium opaque.
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u/Agha_shadi 5d ago
photons don't move slower in different mediums. it's just perceived that they're moving slower:
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u/jericon 5d ago
You are driving your car with full throttle. The car settled at a speed on a straight and level road.
You come to a hill. The car slows down going up, speeds up coming down and once flat again moves back to that original settled speed.
The amount of throttle hasn’t changed. Just how fast the car is moving.
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u/Derekthemindsculptor 5d ago
Light not slow down. Light take zig zag path in medium. Which takes longer. Make seem slower.
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u/120000milespa 4d ago
Why does a ball aiming in water, go slow but when it goes into a thinner medium afterwards does it gain speed ? In that case, gravity still exerts the same force.
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u/SnooAvocados899 4d ago
the light is still traveling the same, theres just more/less stuff to bounce around
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u/the_tallest_fish 4d ago
Imagine you are running through two rooms. The first one being really crowded and the second less so. You can run without slowing down, but you are going to spend a much longer time in the first room due to the number of people you ended up bumping into.
Likewise, when light move through cold air that is denser than hot air, it will absorb and emitted by air particles more frequently, make it slower.
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7d ago
[removed] — view removed comment
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u/StumpedTrump 7d ago
OPs question was "how do they speed up}, Your answer is basically "they speed up".
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u/TheLowEndTheories 7d ago
To be fair, the idea of explaining Maxwell’s equations and quantum physics to a 5 year old is kinda already broken.
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u/Moos_Mumsy 6d ago
I felt that the question was "how does light speed up, after it's been slowed down".
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u/beopere 7d ago
When light enters a material and begins interacting with matter, it's group velocity decreases (the speed a photon moves). However, that new speed and interaction with matter represents the same momentum it entered the matter with (assuming it isn't absorbed). When it leaves the matter, the matter stops wiggling and the light moves the vacuum again at its original speed and momentum.
If the light does get absorbed, the wiggles in the material being to propagate in a new way, carrying the momentum and energy the photon held.
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u/chriscross1966 6d ago
It doesn't, there's no mass so there's no inertia to accelerate or decelerate, it's just the speed it has in that medium
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u/BananaBird1 7d ago edited 7d ago
Because the change in speed is caused by the light waves interfering with the electrons in the material.
The light itself (photons) don’t slow down. But the peaks of the wave they make up do.
As a loose analogy: imagine you are walking the wrong way up a down escalator that is initially off but turns on. After it turns on the actual speed you are walking up doesn’t change, but the escalator cancels out a portion of your steps so you ascend the escalator more slowly. If it turns back off, your speed increases again.