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Photon interaction with atoms in transparent medias

  1. May 12, 2014 #1
    Dear All,
    I have read many posts on this forums about the interaction of photons with the atoms of transparent materials. The point that I was searching for in particular was "why is the speed of light in transparent material less than c".
    The explanations given by many about the absorption and re-emission of photons by the atoms are quite elaborate. However, there is no mention of scientific references for that theory.
    Can anyone, please, provide me with some references where this theory is explained.
    Thank you in advance for your help.
    Milad H.
     
  2. jcsd
  3. May 12, 2014 #2

    DrDu

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    The book by Born and Huang, "Dynamical theory of crystal lattices" is one of the most careful and thorough discussions, but quite high brow. Seems to be available online:

    http://www.chem.elte.hu/departments/elmkem/szalay/szalay_files/KvantKemSzemin/Max%20Born%20%20K%CA%BBun%20Huang%20Dynamical%20theory%20of%20crystal%20lattices%20%201954.pdf [Broken]
     
    Last edited by a moderator: May 6, 2017
  4. May 12, 2014 #3

    UltrafastPED

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  5. May 12, 2014 #4

    UltrafastPED

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    "absorption and re-emission" would result in missing spectral lines, and random loss of coherence because emission occurs at random times and directions. So let's start over:

    For transparent substances (glass, air) we know that an image is propagated by the light passing through that material. For this to happen several things have to be true:

    1. The waves must retain their relative phases (coherence)
    2. The waves must travel in the forward direction (forward scattering)

    Forward coherent scattering only occurs for the light which obeys Snell's law while traveling through the material; Snell's law depends upon the angle of incidence, and the index of refraction for the materials (e.g., air-glass-air).

    For any wave propagation passing from one material to another there is a "Snell's Law"; it is a property of waves, and the change in speed of wave propagation is a measure of "impedance" of the material. This impedance is due to "forward coherent scattering" which delays the phase-front due to interference effects from the different scattering sources (the Huyghens' wavelet mechanism). All directions which are not the "forward" direction described by Snell's law result in destructive interference; in the forward direction you get constructive interference.

    So inside the material the light travels at c, but due to the "forward coherent scattering" a geometric analysis shows that the effective speed of the phase front is reduced ... but once the light crosses out of that material the effects are determined by the new material (air after coming from the glass), and the light has a new characteristic speed. But all along it is travelling at c.

    Richard Feynman explains this in more detail in "QED: The Strange Theory of Light and Matter".

    You can find some more detailed information here:
    http://www.ece.rice.edu/~daniel/262/pdf/lecture11.pdf

    http://www.scribd.com/doc/27753743/C...ght-Scattering
     
  6. May 12, 2014 #5

    DrDu

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    No, it starts already in chapter 8 and basically, the whole book is dedicated to wave propagation in matter.

    Not at variance with your scattering theoretic explanation of the problem I think that it is easiest to understand the problem in a purely classical setting. Namely a dipole driven by an electric field will lag behind the field and the radiation emitted by it will be somewhat out of phase. Hence repeated scattering will lead to a softening of the wave (i.e. a smaller wavelength than in vacuo).
     
  7. May 12, 2014 #6

    morrobay

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    I agree with above, that index of refraction is easiest to understand classically.
    I would like to see a QM explanation also for the index of refraction.
     
  8. May 12, 2014 #7
    Thank you all for your help. I already have Born's book, but to be honest it's quite complicated.
    I found in Feynman's lectures a "gentler to the mind" version.
     
  9. May 12, 2014 #8

    UltrafastPED

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    The QM version is similar to the classical ... the wave function interfers following the scattering events.
     
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