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Light and mediums

  1. May 24, 2004 #1
    1) Why does light "slow down" when it passes through a medium. Is it because the light is reflected off of other particles? Or, is it because the light is bent by the particles? Is there another explanation?

    2) I've heard of experments where light is "stopped." How is it possible to "stop light." Is the light just trapped?
     
  2. jcsd
  3. May 25, 2004 #2
    In the wave picturem light slows down because the wave propagation speed is smaller, due to changes in the permeability and permittivity. In the QM view, photons are absorbed and re-emitted by virtual states in the material - the photons travel at c, but the overall propagation time increases.

    In the experiments where light was slowed dramatically, they were storing the photon information coherently in the spin states of the atoms of the medium (an alkali vapor), and then recreating the light with a second laser pulse. A phenomenon called "electromagnetically induced transparency" was involved.
     
  4. May 25, 2004 #3

    Chi Meson

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    In other words (and in no way contradicting the previous post)...

    as light goes through transparent substances, its energy is temporarily absorbed by the atoms of the substance. The energy is usually released a tiny fraction of a second later. THis is what "slows down" the light. In between the atoms light is travelling (as always) at "c."

    Currently, scientists are working on materials that can hold on to this energy for much longer periods, perhaps even indefinately.
     
  5. May 25, 2004 #4

    ZapperZ

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    OK, so I may be nitpicking a bit here. However, there is a subtle but important issue here.

    If you take a carbon atom and arrange it into a diamond structure, you get a material with a particular index of refraction. Now, take the same carbon atom, and arrange it into a graphite structure, you get a DIFFERENT index of refraction. One can do this with silicon too. What this means is that for the same, identical atom, different arrangements can produce different degree of transparencies. The point here is that optical conductivity in solids isn't usually due to absorption by the "atom" in the material. If it does, then both diamond and graphite would have the same index of refraction.

    What is relevant here is the crystal structure and thus, the phonon modes available in that particular configuration. The optical phonon modes (as opposed to the acoustic mode) greatly influence the optical properties of a material. These lattice vibrations (phonons) are external to the atom, meaning it doesn't involve atomic transitions. In an opaque material, light that impinges on the material are absorbed by the lattice in these vibrations and converted to heat. In a "transparent" material, these phonon modes are available for excitation and retransmission.

    Now this mechanism isn't similar in atomic gas where light has been "slowed down" to 0 m/s. Here, the mechanism is more exotic (if that is possible) than what I've described for solids.

    Zz.
     
    Last edited: May 25, 2004
  6. May 25, 2004 #5

    Chi Meson

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    Zz:
    I really couldn't argue with you. But are you saying that atomic transitions do NOT occur during the absorption/re-emission processes as light transmits through a medium? OR are you saying that there are some interesting exceptions to this traditional description?
     
  7. May 25, 2004 #6

    ZapperZ

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    Within the visible spectrum, I can't think of any solids that involve atomic transitions as a mechanism for optical transmission. This is especially true for clear glass (silicates).

    For other medium, such as rare gasses, we have to look at it case by case, since different parameters (such as photon energies) can dictate which is the more probable mechanism.

    Zz.
     
  8. May 25, 2004 #7

    Chi Meson

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    Zz:

    So my previous explanation, would you characterize your response to it as
    "well, it's not quite like that" or
    "Oh, my god, that is simply the wrong way to think about it!"

    Since I teach high school students, I am always trying to get the balance right between presenting models that are simple enough to get through to beginners, but not too simplified so that they are flawed.

    The atomic transitions have been presented as the model for light propagation through solids for a long time. Is your description recent? Or have the textbooks just been slow on getting this right? (I am assuming that what you present is currently widely accepted)
     
  9. May 25, 2004 #8

    ZapperZ

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    Let's just say that I would not use the "atomic transition" explanation to explain about visible light propagation through solids. Without even evoking atomic vibration, etc., that explanation by itself is inconsistent with the carbon example that I have given.

    Just tell those spoiled brats that the atoms in the solid vibrate and retransmit the light. <of course you know that I say this with the utmost love for your students> :)

    Zz.
     
  10. May 25, 2004 #9
    thanks for the help
     
  11. May 26, 2004 #10

    Chi Meson

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    TUrns out, as I went through my collection of textbooks, none of them specifically say "atomic transition" when describing transparency. THey are all specifically vague (ya like that oxymoron?) in their descriptions.

    Thanks for catching the inaccuracy; you've helped me and untold hundreds of my future students.
     
  12. May 26, 2004 #11

    ZapperZ

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    You're welcome. We all learn something on well-run sites such as this, no matter how old or how much we already know.

    I also want to say that you are at the frontline as far as physics education goes. You have a direct influence on whether a student find physics fascinating, or get completely turned off by it. Please know that your effort in introducing physics to so many kids (and making it fascinating enough that they stay awake) is appreciated.

    Zz.
     
  13. May 26, 2004 #12

    Integral

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    ZZ,
    I want to add my thanks for your excellent explanation. I have never been real comfortable with light propagation in transparent media, you have helped a lot.
     
  14. Jul 6, 2004 #13
    Are u sure this is true? Eventhought the change of the structure of the material do result in change of refraction index but this does not lead to the fact that the structure is the only factor influencing. Do you have any actual fact or proof that the atom is not involved?

    The other thing is why the light will be slowed down to 0 m/s in atomic gas?

    Helps and guidance is very appreciated. Thx
     
  15. Jul 6, 2004 #14

    ZapperZ

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    Considering that (i) the atoms in the different structure are identical; (ii) the structure is the only thing that changed; and (iii) the index of refraction is different for the two, it is logical to conclude that the structure difference is the major cause for the change in index of refraction, and that the atoms play no significant role.

    In fact, even within the SAME material with the SAME structure, the index of refraction can be different along different crystal orientation. Many other properties, such as the work function, can have different physical values along different crystallographic direction. Thus, the crystal structure of the material is certainly the predominant parameters here in many aspect of the material. It is why, for instance, that optical conductivity experiment in condensed matter physics is a powerful tool to study the phonon structure of a material - optical phonon modes are the ones being excited by light transmission through the solid.

    This is a completely different beast. Cooled atomic gas that can be made transparent via an external EM field has been shown to be able to "hold" photons for an indefinite length of time. This was shown several years ago by the Lena Hau's group at Harvard.[1] I say that this is a different mechanism because here you have atomic gas, whereas in ordinary solids, you have atoms/molecules in a crystal structure. The phonon modes in the latter are a lot more dominant in dictating the solid's properties over the normal range of parameters.

    Zz.

    [1] C. Liu et al., Nature v.409, p.490 (2001).
     
  16. Jul 6, 2004 #15
    Good subject:

    Could this be caused by light redirection and change in density?
     
    Last edited: Jul 6, 2004
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