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What really cause light to appear slower in media?

  1. Jul 1, 2011 #1

    EHT

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    Some textbooks that I read explain it in a way kind of like this:
    In a material the photons is absorbed by an atom and then re-emitted a short time later, it then travels at a short distance to the next atom and get absorbed&emitted again and so on. How quickly the atoms in a material can absorb and re-emit the photon and how dense the atoms decides the speed of light in that material. So the light appears slower because it has a smaller “drift speed”.

    But recently I realize an alternative explanation:
    Atoms respond to the light by radiating electromagnetic wave. This “new light” interferes with the “old light” in some way that result in delayed light, consequently effectively the light covers a smaller phase each second. Which gives the impression of a lower phase velocity . However the group velocity is changing in a complicated way.

    I think that the first explanation does not explain the change in phase velocity of light. if we consider light travelling into a slab of negative refractive index non-dispersive material, let’s say the light is directed perpendicular to the slab. The phase velocity’s direction will be flipped, but group velocity’s direction in the material will not change. Only the second explanation can explain the flipped phase velocity direction. I guess that the velocity that we get in the first explanation is actually belongs to the group velocity. It makes sense to me that the front most of the photon stream determines the first information that the light delivers. Am I right?
     
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  3. Jul 1, 2011 #2

    Vanadium 50

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    You can read the explanation in the FAQ in the General Physics Forum.
     
  4. Jul 2, 2011 #3

    EHT

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    I have read it, but it doesn't really answering my question. I already agree that the absorption-emission process isn't related with the atomic transition. I was thinking that the media is a fluid, I thought that there will be no such fine pattern as in the solid, therefore the phonons' effect will not be important. And for the absorption process I was talking about a non-energy shifting or an elastic absorption, which gives delay too, but I don't know whether it is significant or not. I don't really understand about this, would you mind to explain?

    Despite of the absorption-emission process. What I want to ask is what really cause the phase velocity of light to be decreased?
    1. decreased "drift velocity" of photons ( they aren't the same photons, they are re-emitted all the time)
    or
    2. phase difference between absorbed and emitted light
    or
    something else
     
  5. Jul 5, 2011 #4
    The absorption-emission picture of electromagnetic waves traveling through a dielectric medium is just a beginner's visualization model. The wave is interacting with all the matter in its path, not just a single atom. So a better visualization is that of someone running through mud. The runner is not continually destroyed and recreated. Rather, he is slowed down when the interaction with surrounding material is stronger.
     
  6. Jul 5, 2011 #5

    EHT

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    @chrisbaird during the silence of this thread, I already upgraded my understanding. Now I understand that in the wave point of view, the light is not absorbed, the light is just interfering with the new light waves emitted by the atoms that result in delayed light(advanced in phase),this can easily shown by using simple phasor diagram. Consequently effectively the light covers a smaller phase each second. Which then gives the impression of a lower phase velocity. This wave point of view is corrrect for every wavelength. And for the case of light's wavelength larger than atom's spacing, if we view the light as photons, the photons will be absorbed and re-emitted by the atoms as a group (phonons). My problem is if the light's wavelength is smaller than the distance between neighboring atoms, then one photon will only collide with one atom at a time. Therefore we need another absorption mechanism involving quantum mechanics. Is it primarily governed by spontaneous or stimulated absorption/emission?or else?
     
    Last edited: Jul 5, 2011
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