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Question about slowdown of the speed of light in solid medium

  1. Feb 8, 2008 #1
    I have read the sticky topic (FAQ of Physics Forums), concretely the topic "Do Photons Move Slower in a Solid Medium?" five or six time, and I would ask you about this part:
    How it is re-emitted, when it is not first absorbed?
  2. jcsd
  3. Feb 8, 2008 #2
    It was absorbed for a short time, is what it is saying. So it is absorbed, just not indefinitely. It is absorbed then re-emitted almost instantly, causing the delay.
  4. Feb 8, 2008 #3
    Yes, this can be slightly confusing if not understood well; it means that the atoms don't make a transition between different energy levels but that they simply "resonate" according to the em radiation hitting on them and then they re-emits the same em wave (but with a slight delay). Think of a sort of "reflection" of a light beam on a mirror: if the reflection is perfect, the mirror doesn't absorb any energy from the beam, even if the em wave do interact with the mirror's atoms.
    "Absorbed" in that contest means exactly this, that is, "energy is not absorbed from the em wave"; it doesn't mean that "there is no interaction"; so "re-emitted" about photons in this case means that they do interact but without energy absorption.
    Last edited: Feb 8, 2008
  5. Feb 8, 2008 #4
    It was absorbed and released, so why they don't support the theory of electron transitions? Exactly that happens, are they want to say that there is chain reaction (there must be more than 1 atom) to be refraction successful?
  6. Feb 8, 2008 #5
    I made the example of reflection, I hoped to be explicative. You could think of sun's light scattering by the air: light interacts with the air's molecules and atoms, but it's not absorbed (its energy is not enough to make them do energy transitions). The prove that it really interacts with air is the blue colour of the sky.
  7. Feb 8, 2008 #6
    So if it is not enough to make them do energy transitions, how the light interacts with them?
    Can the light just pass through them? But where is the time delay then?
  8. Feb 8, 2008 #7
    Anybody knows?
  9. Feb 8, 2008 #8


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    "So the lattice does not absorb this photon and it is re-emitted but with a very slight delay. This, naively, is the origin of the apparent slowdown of the light speed in the material. The emitted photon may encounter other lattice ions as it makes its way through the material and this accumulate the delay."

    That explanation is not correct. The wave length of light is about 1000 times the distance between molecules, so a single photon interacts with thousands of molecules at the same time. It does not get absorbed and re-emitted by individual molecules. The treatment in most textbooks, showing the increase in permittivity due to macroscopic polarization, is correct.
  10. Feb 8, 2008 #9


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    The explanation being given, as stated, is the most naive scenario. If you look at the FAQ, there is a clear mention of the phonon modes, which, by defintion, is a collective phenomenon. That is what is meant by "lattice ions". It isn't about individual molecules or atoms.

  11. Feb 8, 2008 #10
    And how does than the light is slowed down?
  12. Feb 8, 2008 #11
    Yeah, he spends most of the article explaining that the conception quoted in the OP is not correct.

    Great article ZapperZ. Lattice vibrations are quantized? ❊brain explodes❊ This site is so great.

    Is there something physical, or some area of effect, that's physically the same size as the wavelength?
  13. Feb 8, 2008 #12
    What about my question?
  14. Feb 8, 2008 #13
    I'm not a physicist myself but it seems to me that you'd need to achieve some understanding of phonons to better comprehend ZapperZ's article.
  15. Feb 8, 2008 #14
    ZapperZ can you tell me concretely what happens? Are the phonons (the lattice of ions) absorbing and then re-emiting the energy?
  16. Feb 9, 2008 #15
    If the electrons absorb the energy of any portion of the visible spectrum, the light that transmits through will appeared colored according to the portion of the spectrum absorbed. In fact, the color of any object is a direct result of what levels of energy the electrons in the substance will absorb!
    So what actually happens here?
  17. Feb 9, 2008 #16
    Just from my own point of view not understanding this myself, it may more successfully encourage people to answer you if you ask questions that demonstrate that you're making a serious attempt to understand what phonons are and how they fit into this topic.

    (Or, it might just be something that can't be boiled down to as simple a form as you want it to be.)
  18. Feb 9, 2008 #17
    I think it is not so much complicated. Just so much information confuses you.
    I found this site first saying that:
    I found other article on the same site:
    Again, the same site:
    So what is actually correct!?!?
  19. Feb 9, 2008 #18
    The first refers to transparent materials (like glass), the second to conductors (like metals).
  20. Feb 9, 2008 #19
    Actually all are for one same thing. On the third link, just below the quoted text, see there is Law of Reflection. Does anybody know what is correct?
  21. Feb 9, 2008 #20
    Probably it's thinking about photons that confuses you, so you could prefer not to think about photons at all, since Maxwell's equations are enough to treat the phenomenon. The light's velocity we are talking here is what it's called "phase velocity" of a wave. When an em wave interacts with atoms without being absorbed, the atom or the atoms collectively (as in the case of glass) oscillate in response of that wave, so generating another, second, wave which is not in phase with the first one, but with a slight phase delay; this then will make the near subsequent atoms oscillate which then will emit another, third wave, with a slight phase delay with respect to the second and so you have a phase delay proportional to the light path inside the material. Now remember that we are talking about "phase velocity" and you understand (I hope! :smile:) why light's speed is slowed down inside the glass.
    A link about waves animations:
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