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    Default Speed of Light in a Medium

    Can someone explain to me, in layman's terms, why light slows down in a medium?


    I understand that in some exotic forms of matter photons are actually absorbed and then released at a later point, but I don't think that is what is normally happening.


    Do the individual photons actually slow down when passing through a medium, or are they merely prevented from taking a direct route yet still traveling at C? (Like, if I am driving my car at a constant speed of 10 miles an hour, it can take more more than 10 hours to reach a destination 100 miles away if the road winds back and forth).
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    Default Re: Speed of Light in a Medium

    It actually slows down. If it was about scattering (so having to change direction from time to time and thus taking a longer route) than the light would be distorted as scattering is fairly random. So nicely transparent materials like glass could not have a high refractive index (which indicates a significant change in light speed) and they do.

    So now we need to consider why photons slow down like that? Even if they are not absorbed, they still interact with charged particles inside a given material, which results in a changed speed of propagation. To go any further, we would need to look into equations governing electrodynamics.
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    Default Re: Speed of Light in a Medium

    Quote Originally Posted by Talakeal View Post
    Can someone explain to me, in layman's terms, why light slows down in a medium?


    I understand that in some exotic forms of matter photons are actually absorbed and then released at a later point, but I don't think that is what is normally happening.


    Do the individual photons actually slow down when passing through a medium, or are they merely prevented from taking a direct route yet still traveling at C? (Like, if I am driving my car at a constant speed of 10 miles an hour, it can take more more than 10 hours to reach a destination 100 miles away if the road winds back and forth).
    "explain" and "in layman's terms" are in some tension against one another on this topic. Also, asking what "actually" happens to "individual photons" could get very philosophical in a hurry.

    In any case, the collective behavior of the photon fields passing through a transparent medium is that the photon fields do slow down together. The classical description of the index of refraction in terms of relative permittivity and permeability ends up being an excellent approximation of what happens to the quantum photon fields when they interact with the phonon fields that define the possible vibrational modes of a solid.
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    Default Re: Speed of Light in a Medium

    I just covered this in class, so I might be able to help. When light travel through matter the photons get scattered. This scattering releases wavelets which interfere and form a new wave which lags or leads the first wave. These two waves interfere, which forms a new wave out of phase with the original. When this light is compared to light in a vacuum, the phase shifted wave either leads the vacuum wave or lags behind it, creating the sense that the wave is a different speed than the original.

    If it helps, I have a picture. The purple wave is out of phase with the black one, and seems to be slower because it completes its first cycle after the black one.
    Spoiler: Wave Picture
    Show


    In conclusion, the photons do not change speed. It is the interference which causes the sense of change. I did just learn this though, and I have yet to take a quantum class, so my explanation may not be completely right.

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    Default Re: Speed of Light in a Medium

    Quote Originally Posted by AstralSeal View Post
    I just covered this in class, so I might be able to help. When light travel through matter the photons get scattered. This scattering releases wavelets which interfere and form a new wave which lags or leads the first wave. These two waves interfere, which forms a new wave out of phase with the original. When this light is compared to light in a vacuum, the phase shifted wave either leads the vacuum wave or lags behind it, creating the sense that the wave is a different speed than the original.

    If it helps, I have a picture. The purple wave is out of phase with the black one, and seems to be slower because it completes its first cycle after the black one.
    Spoiler: Wave Picture
    Show


    In conclusion, the photons do not change speed. It is the interference which causes the sense of change. I did just learn this though, and I have yet to take a quantum class, so my explanation may not be completely right.
    That just sounds wrong, what level and subject of class was this? Refraction is due to the wavelength of the light changing because the speed changes (the frequency stays the same). Quite why the speed changes I don't know, but if it was due to interference or interactions between photons I would expect to see blurring, and you don't see blurring when you look through clear, clean glass or water.
    Last edited by halfeye; 2021-10-23 at 06:36 PM.
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    Default Re: Speed of Light in a Medium

    The level was university and the subject was physics (optics specifically). I admit that it might be a somewhat simplified because I am still a lower year student, but it is how it was explained. I might be wrong about the scattering off the atoms, though the lecture mentioned wavelets interfering destructively as they moved backwards and to the sides, leaving only a wave heading forward. I think the part about two waves interfering to form a new, phase-shifted wave is supposed to be how it works though, even if the scattering part is incorrect.

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    Default Re: Speed of Light in a Medium

    https://www.youtube.com/watch?v=CUjt36SD3h8

    As mentioned above, laymen's explanations designed to simplify the process leave out a good chunk of quantum weirdness that's key to understanding the process.

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    Default Re: Speed of Light in a Medium

    Quote Originally Posted by AstralSeal View Post
    The level was university and the subject was physics (optics specifically). I admit that it might be a somewhat simplified because I am still a lower year student, but it is how it was explained. I might be wrong about the scattering off the atoms, though the lecture mentioned wavelets interfering destructively as they moved backwards and to the sides, leaving only a wave heading forward. I think the part about two waves interfering to form a new, phase-shifted wave is supposed to be how it works though, even if the scattering part is incorrect.
    The idea that every point on a wavefront is radiating comes up in some contexts. I don't think "the photons scatter off the atoms and create new waves" corresponds to anything physical in a transparent medium, but I could be wrong. My guess is that it very poorly explains polarization effects in materials, which is one of the crucial things Fresnel did that shifted optics from the particle basis of Newton to the (almost) modern wave optics.

    Upthread someone mentioned interaction with ions and charged particles. Since the speed of light in a medium depends on the permitivity and permeability of the medium I suspect that such is a decent beginning point. Permitivity and permeability are measures of how a material responds to magnetic and electric fields after all.
    Last edited by Rockphed; 2021-10-23 at 11:36 PM.
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    Default Re: Speed of Light in a Medium

    I believe a decent simplification is that photons passing closely to other particles interact with them through the electromagnetic field (wavefunctions overlapping something something), which creates a kind of "drag".

    How that makes light beams bend when they move between mediums is still a total mystery to me, though.
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    Default Re: Speed of Light in a Medium

    Quote Originally Posted by Anymage View Post
    https://www.youtube.com/watch?v=CUjt36SD3h8

    As mentioned above, laymen's explanations designed to simplify the process leave out a good chunk of quantum weirdness that's key to understanding the process.
    Thank you!


    He does debunk the two explanations I have heard before and does raise the same problems I had with them, so there is that.

    But his explanation seems incomplete and contradictory, and now I am more confused than before. The only way I can see his explanation working is if it is one of the two debunked theories, only applied to fields and waves rather than particles.

    Further research shows that while we mathematically understand what is happening, perhaps we don't understand it conceptually.


    I did find the important bit though; C never changes, and that even if photons get slowed down by a medium, other mass-less things do not.
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    Default Re: Speed of Light in a Medium

    Quote Originally Posted by Yora View Post
    I believe a decent simplification is that photons passing closely to other particles interact with them through the electromagnetic field (wavefunctions overlapping something something), which creates a kind of "drag".
    I suspect it's gotta be something like that, and the molecular structure is presumably important because diamond which has a very strong structure has a very high refractive index.

    How that makes light beams bend when they move between mediums is still a total mystery to me, though.
    This one I know about, I was taught it for the "A" level (immediately pre-university level), it's a diagram you can draw it if you have to.

    Wikipedia has it:

    A correct explanation of refraction involves two separate parts, both a result of the wave nature of light.

    Light slows as it travels through a medium other than vacuum (such as air, glass or water). This is not because of scattering or absorption. Rather it is because, as an electromagnetic oscillation, light itself causes other electrically charged particles such as electrons, to oscillate. The oscillating electrons emit their own electromagnetic waves which interact with the original light. The resulting "combined" wave has wave packets that pass an observer at a slower rate. The light has effectively been slowed. When light returns to a vacuum and there are no electrons nearby, this slowing effect ends and its speed returns to c.
    When light enters, exits or changes the medium it travels in, at an angle, one side or the other of the wavefront is slowed before the other. This asymmetrical slowing of the light causes it to change the angle of its travel. Once light is within the new medium with constant properties, it travels in a straight line again.

    https://en.wikipedia.org/wiki/Refrac...al_explanation

    It may marginally help if you draw it yourself (without the movement). It's just that the gaps between the waves get shorter in the slower medium, so to keep the wavefronts connected, the direction of travel has to change.
    Last edited by halfeye; 2021-10-24 at 11:36 AM.
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    Default Re: Speed of Light in a Medium

    Quote Originally Posted by halfeye View Post
    When light enters, exits or changes the medium it travels in, at an angle, one side or the other of the wavefront is slowed before the other. This asymmetrical slowing of the light causes it to change the angle of its travel. Once light is within the new medium with constant properties, it travels in a straight line again.
    And that doesn't make sense to me. A single photon still should have a bend path. The stream of photons in a beam of light are not a single object.
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    Default Re: Speed of Light in a Medium

    Quote Originally Posted by Yora View Post
    And that doesn't make sense to me. A single photon still should have a bend path. The stream of photons in a beam of light are not a single object.
    Single photons don't typically follow what you'd call a single "path."

    A single photon can, and usually does, travel like a wave.

    See the double slit experiment, for example.
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    Default Re: Speed of Light in a Medium

    That's basically what I heard. It takes multiple paths, some of them rather longer than a direct path at the new angle would be, something something path-integral, and the overall path of the photon is longer than it would be otherwise
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    Default Re: Speed of Light in a Medium

    Quote Originally Posted by Yora View Post
    And that doesn't make sense to me. A single photon still should have a bend path. The stream of photons in a beam of light are not a single object.
    A single photon still has a wavelength. So it behaves like a wave. Which in this case means its path bends.

    The bend in the beam is because of the waves, if it was just one wave, it would bend the same, it just wouldn't be half so clear that the change in velocity was why it bent.
    Last edited by halfeye; 2021-10-24 at 03:32 PM.
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    Default Re: Speed of Light in a Medium

    I just realized, I may have been way overthinking this.

    Let me ask a hypothetical:

    I have a cube of plane with a refraction index of 1.33 that is 1LY across and arbitrarily tall and wide. It is somehow floating in an endless void. I shine a light into it. How long does it take for the beam of light to exit the other side?
    Last edited by Talakeal; 2021-10-24 at 04:58 PM.
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    Default Re: Speed of Light in a Medium

    The speed of light in water is about 75% as in vacuum, so 1 lightyear takes about 16 months instead of 12.
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    Default Re: Speed of Light in a Medium

    Quote Originally Posted by Yora View Post
    The speed of light in water is about 75% as in vacuum, so 1 light year takes about 16 months instead of 12.
    If that is correct, then yes I was over thinking it and understand it now.


    But I have a suspicion that I don't understand it and the correct answer is ~21.3 months...
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    Default Re: Speed of Light in a Medium

    Though that is ignoring General Relativity. A sphere of water a light-year in diameter would collapse into a black hole.

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    Default Re: Speed of Light in a Medium

    Quote Originally Posted by DavidSh View Post
    Though that is ignoring General Relativity. A sphere of water a light-year in diameter would collapse into a black hole.
    Yeah. Spherical cows and all that.
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    Default Re: Speed of Light in a Medium

    Quote Originally Posted by Talakeal View Post
    If that is correct, then yes I was over thinking it and understand it now.

    But I have a suspicion that I don't understand it and the correct answer is ~21.3 months...
    That would be somewhere around 55% of c.

    Of course, when a beam hits a cube at an angle, the distance for the beam to the other side would be different from the cube's side length, and refraction would change the line even further. But on a basic level it's that simple: When a photon passes through a transparent material, it will move at a fixed percentage of c, based on the atoms of the material.

    While we're at it, that's where the blue Cherenkov radiation comes from when radioactive material is put in water. The electrons emitted by radioactive decay can come out with enough momentum to pass through water at speeds like 90% c, while the light produced when the electrons hit a water molecule can only move at 75% c. The effect is very much analogous to sonic booms, but since the electrons crashing into water molecules release photons, the result is a glow of light instead of a soumd.
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    Default Re: Speed of Light in a Medium

    Quote Originally Posted by Yora View Post
    Of course, when a beam hits a cube at an angle, the distance for the beam to the other side would be different from the cube's side length
    This is correct.

    , and refraction would change the line even further.
    That second part of that is mistaken. If the refractive index of the material nearer the light source is lower than the refractive index of the other material, the refraction will be toward the perpendicular to the surface. The reverse is how total internal reflection happens, when light comes from a higher refractive index to a lower, it can come at such an angle that it cannot be transmitted through the surface.

    https://en.wikipedia.org/wiki/Total_internal_reflection

    Notice that in the Wikipedia diagrams where there are rays from the water to the air, the rays in the water are less bent than the ones in the air, this also applies in reverse, but obviously the rays that never reach the air can't be reversed from the air.
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    Default Re: Speed of Light in a Medium

    Fun fact: if you have a long enough piece of coax you can measure the way a structure influences the speed of light. And pulses really do take different times in cables with different fillers. Cables (and other waveguides) take longer for light because the light doesn't go straight through but bounces around.
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    Default Re: Speed of Light in a Medium

    Quote Originally Posted by halfeye View Post
    That second part of that is mistaken. If the refractive index of the material nearer the light source is lower than the refractive index of the other material, the refraction will be toward the perpendicular to the surface. The reverse is how total internal reflection happens, when light comes from a higher refractive index to a lower, it can come at such an angle that it cannot be transmitted through the surface.

    https://en.wikipedia.org/wiki/Total_internal_reflection

    Notice that in the Wikipedia diagrams where there are rays from the water to the air, the rays in the water are less bent than the ones in the air, this also applies in reverse, but obviously the rays that never reach the air can't be reversed from the air.
    Refraction might bend the beam to an angle that is shorter than if it wasn't bend, but it's a further factor that affects the path from the entry to the exit point. It could also make it longer, since we're talking a cube and not a sphere.
    Last edited by Yora; 2021-10-25 at 04:51 AM.
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    Default Re: Speed of Light in a Medium

    Quote Originally Posted by Yora View Post
    Refraction might bend the beam to an angle that is shorter than if it wasn't bend, but it's a further factor that affects the path from the entry to the exit point. It could also make it longer, since we're talking a cube and not a sphere.
    To amplify on this, if the non-bent exit point was on the opposite face (e.g. passing from the 1 to the 6 on a D6), the bending would make the path shorter. If the non-bent exit point was on an adjacent face (e.g., passing from 1 to 2), and the bent exit point was still on that adjacent face, the bending would make the path longer. Otherwise you'd have to run the numbers.

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    Default Re: Speed of Light in a Medium

    Quote Originally Posted by DavidSh View Post
    To amplify on this, if the non-bent exit point was on the opposite face (e.g. passing from the 1 to the 6 on a D6), the bending would make the path shorter. If the non-bent exit point was on an adjacent face (e.g., passing from 1 to 2), and the bent exit point was still on that adjacent face, the bending would make the path longer. Otherwise you'd have to run the numbers.
    Yeah, I get that.

    Quote Originally Posted by Yora View Post
    Refraction might bend the beam to an angle that is shorter than if it wasn't bend, but it's a further factor that affects the path from the entry to the exit point. It could also make it longer, since we're talking a cube and not a sphere.
    If you're saying the same as the guy above then yeah, I wasn't really considering the "through the sides" options. Working out the exact angles would be a maths nightmare for me, almost certainly involving sine, probably cos and maybe tan, but the trends are easy.
    Last edited by halfeye; 2021-10-25 at 11:14 AM.
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    Default Re: Speed of Light in a Medium

    Quote Originally Posted by halfeye View Post
    If you're saying the same as the guy above then yeah, I wasn't really considering the "through the sides" options. Working out the exact angles would be a maths nightmare for me, almost certainly involving sine, probably cos and maybe tan, but the trends are easy.
    Well, the refraction angle already uses sines or cosines. I think it is that the ratio of the sines of the angles is the same as the ratio of the refractive indices. There could be a square or square-root in their too. I should probably just look it up on wikipedia before mouthing off, but when has actually bringing facts into a discussion actually helped?
    ...
    Okay, looked at wikipedia. No squares or square-roots. That must be from the speed of light in a medium being the square-root of the inverse of the product of the permittivity and permeability.
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    Default Re: Speed of Light in a Medium

    Okay, so imagine you have a slinky with one end in your hand and the other tied up far away (or don't imagine, buy one and try this at home).

    You can pretty easily send a wave to the other side.

    You can move the one end in other ways than a wave, but that's not going to produce a motion that's going to carry on traveling without further input. Although you may accidently produce waves by jerking it around randomly.

    A photon is a wave of the electromagnetic field. It is not the only way the field can move, or the smallest unit. However, it is the smallest unit that can travel a large distance without further input. Bits of motion will tend to "settle" into waves that will travel off.

    Glass is full of strong electric and magnetic fields. Those fields add up to zero, but the absolute value of them is high.

    So when the photon hits the glass, it pushes and pulls on the electrons in the glass. The disturbed electrons push on the EM field. The disturbances in the EM field (very likely) now resemble the photon if it was delayed and traveling at a different angle.

    Is it the same photon? Since this all happens so fast, the conventional thing is to say it is.

    Another way to put it is that part of the photon is absorbed by the glass and re-emitted.

    Another way to put it is that the as the photon travels through the glass, it becomes a wave of not just the EM field, but also the electron field (as the electrons is the glass are moving as part of the wave), and is thus a massive particle.
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