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Speed of light in transparant materials

  1. Sep 26, 2009 #1
    In his second Auckland lecture, Feynman said that light doesn't slow down in materials, it still goes at c in the vacuum between molecules. It just gets [and I don't remember exactly what he said but something like] it just gets waylaid when it passes near matter, or maybe he said it is absorbed then remitted.


    Why does light slow in a transparent material, anyway? Is it that the photon passes so close to atoms that they are slowed by very local gravitational redshift?

    Or what?

    Also, what is the name for time slowing near mass? Gravitational redshift? Time dilation? Fourth-dimensional Lorenz contraction?

    Thank you. You are the priests of my religion.

    -- faye kane, idiot savant
  2. jcsd
  3. Sep 26, 2009 #2


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    Please read the FAQ thread in the General Physics forum.

  4. Sep 27, 2009 #3


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    Gravitational redshift plays no significant part in the speed of light through a medium. Electrons (mostly) induce a virtual particle "cloud" in molecules, which is the primary contributor to slowing down light in a medium. You wouldn't want to think of the cloud as causing light to be slowed due to actual absorption and re-emission. However, that possibility is in fact a contribution to the effect. Might be better to think in terms that ALL of the possibilities (paths) contribute to the net result.

    ZapperZ is correct that a lot of this kind of information is available in the General FAQs.
  5. Sep 27, 2009 #4
    Hi, I read the section about this question in the FAQ which provides the model of photons interacting with the phonon modes in the solid and getting absorbed. I have a couple questions: 1) What determines the spectrum of a phonon (how does an absorption bandwidth allow a continuous spectrum to pass through?) and 2) how is this consistent with the virtual particle cloud DrChinese mentioned? -Thanks
    Last edited: Sep 27, 2009
  6. Sep 27, 2009 #5
    Instead of being absorbed and re-emitted, the photons may choose a zig-zag way between atoms without interaction (as a more probable process). I do not know why Feynman did not mention this mechanism - he is the author of the path integral. This explains the slower velocity too.

    And if you want a serious answer, think of a media as an ensemble of charged particles. They move in the incident field and radiate so the resulting filed has sources. Obviously in presence of sources the resulting field will "propagate" differently. That is why the Maxwell equations in a medium are different.
  7. Sep 28, 2009 #6
    OOPS! Sorry. I assumed the FAQ was "don't post pirate software or say racist things" like everywhere else. But it's good that there's a condensed list of the answer to probably every question I came here to ask!


    -- faye kane, padwan among Jedi
  8. Sep 28, 2009 #7


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    You need to understand that if there is a phonon modes for absorption, there's no transmission. So in a transparent medium, that mode doesn't quite exist so that vibration corresponding to the light's frequency can't be sustained by the lattice. So in that simplified model, the "dipole" in a chain retransmit that light right away.

    The spectrum of the phonon is determined not only by the elements that make up the bulk material, but also the way in which these elements are arranged, i.e. the crystal structure. The example of carbon versus graphite is a very good example, where both are made up of carbon atoms, but with different crystal structures.

  9. Sep 28, 2009 #8
    Okay, cool, but now I'm curious about: if the photon is absorbed and reemitted by the phonon, what is the mechanism of that if it doesn't involve electrons?

    Someone mentioned path integrals. Is the delay due to the photon taking a longer path? If so, what happened to (what one would think) is the shortest path, straight through? Is it canceled at the quantum wave level, leaving only the longer paths to integrate?

    And if it is canceled, doesn't that mean that whatever it encountered (presumably the phonon), also lost energy because had the photon not happened, the phonon would not have had part of its energy nullified?

    If so, then wouldn't the photon which passed through unaffected (but delayed) have to have gained energy because the phonon lost energy?

    Also, if a solid is balls connected by springs, is the phonon a compression wave that passes through the material via the springs, reflects off the side, then passes through again in the other direction?

    And if the phonon is a vibration through the atomic array, can there be more than one phonon in a material, like multiple vibration modes in your eardrum when you listen to music?

    And finally, is temperature always phonons, or is there chaotic motion too?

    -- faye kane, padwan among Jedi

    Uh-oh, "it" is happening again. It's 7 am and I'm going to Wikipedia to look up "phonon" now. If past is indeed prologue, by 4 pm I will have learned all about phonons but have produced not a single lick of productive work! :smile:
  10. Sep 29, 2009 #9

    Meir Achuz

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    The notion of light being absorbed and re-emitted by atoms (even if proposed by Feynman) does not take into account that the wave length of light is about 5,000 times longer than the distance between atoms. This means that the usual classical textbook explanation of light propagation as a wave motion holds for either classical light or photons.
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