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Photons transmitted through transparent substances.

  1. Jun 20, 2011 #1
    I have noticed in the past that light travels through water relatively undisturbed (I have noticed that there seems to be some small amount of light reflected, but most seems to pass directly through).

    How does this happen, in terms of photons encountering particles?

    I once thought that the correct explanation might be that the photons simply do not interact with the particles of the water. But I think that this must be wrong, because the light changes it's speed/direction upon entering the water, suggesting that some interaction takes place.

    I now imagine a photon entering the water, striking an electron and being absorbed, then being re-emitted by the same electron only to be absorbed by another electron in perhaps the next adjacent atom/molecule. I imagine the process repeating itself from molecule to molecule until the light reaches the other side of the column of water.

    Is my mental model of the process anywhere close to being realistic? If not, is there a better way to imagine it.
  2. jcsd
  3. Jun 20, 2011 #2


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    I think there is a model on your description, but I don't think that is accurate. Maybe someone can elaborate?
  4. Jun 20, 2011 #3


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    ZapperZ has a very useful entry that at least partially answers this question in the Physics forum FAQ.

    https://www.physicsforums.com/showthread.php?p=899393 [Broken]
    Last edited by a moderator: May 5, 2017
  5. Jun 20, 2011 #4
    I think the following portion is the key to the model that ZapperZ describes, as it applies to my problem.

    "On the other hand, if a photon has an energy beyond the phonon spectrum, then while it can still cause a disturbance of the lattice ions, the solid cannot sustain this vibration, because the phonon mode isn't available. This is similar to trying to oscillate something at a different frequency than the resonance frequency. 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." - Posted by ZapperZ in FAQ.

    Although this model is intended to describe a solid (i think), it seems to me that it could also possibly describe a liquid in a similar way (perhaps the molecules are close enough to allow molecules to have some influence on each other, creating a spectrum of vibrational modes which would allow for absorption of a wide spectrum of light.)
    Last edited by a moderator: May 5, 2017
  6. Jun 20, 2011 #5
    What about the portion of photons that are reflected? Would this be a condition where photons are absorbed by electrons in surface molecules and re-emitted back in the opposite direction, instead of being transferred forward/through to an electron in an adjacent molecule? (Sort of like dropping something heavy on a trampoline, and having it reflect back at you)
  7. Jun 21, 2011 #6


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    Yes, but again this is not from absorption/re-emission so much as the qualities of the "lattice". Just remind yourself that electrons and protons are themselves very small (of course this is somewhat relative - but their constituents are considered points) so much so that you may as well consider them a vacuum if you ignored field-like effects. As ZapperZ says:

    "Almost all of the properties of solids that we are familiar with are the results of the collective properties of the solid as a whole, not the properties of the individual atoms. The same applies to how a photon moves through a solid." Or reflects, for that matter.
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