Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Speed of light in a vacuum

  1. Jan 19, 2012 #1
    Why is the speed of light slower in certain substances than in a vacuum. Since most of the atoms is empty, shouldnt the speed be the same? Is it because photons bounce off the nucleus, or because they get re-emitted?
  2. jcsd
  3. Jan 19, 2012 #2


    User Avatar

    Staff: Mentor

    https://www.physicsforums.com/showthread.php?t=511177 [Broken]

    If you have further questions, feel free to ask... this is just to get you started.
    Last edited by a moderator: May 5, 2017
  4. Jan 19, 2012 #3
  5. Jan 19, 2012 #4
    (not the original poster)
    Thank's, that was interesting... but it sort of lead to more questions than answers (do we need phonons to explain the phenomenon ? what are phonons ? and anyway.. what happens ?)
    This is something I have been wondering about myself for a while and hence my hijacking of the thread.
    I got sort of satisfied with Feynman's take on explaining it as long as considering light as many many many photons (or better said as a wave) exciting the medium and ultimately interefering with its offspring (so to speak) so as to generate a new wave that does look like a wave propagating at a different speed. this explanation is satisfying both qualitatively and quantitatively.
    but it is not all that clear if we want to get down enough and just talk about 'one photon'
    It looks like your linked post adresses this precise case but it is not clear enough for me. I interpret it like this:
    -> in some conditions the photon will be absorbed and reemitted (delay)
    -> in some others the photon will just get through
    -> overall the photon will be slowed down
    I'm not sure I interpreted this correctly but I'd like to be corrected on the interpretation, and above all, I would really appreciate if the correction (or confirmation) could come with quantitative arguments.
    exactly what is supposed to be the absoption/emition delay ? is it statistically confined but overall unknowable ? is the final slowdown effect meaningless without statistivally meaningful quantities of photons ? (this is what I currently suppose but I just imagine this is so and would very much apreciate a confirmation just as well as a correction of it with some good explanations :))
    thanks again.
    Last edited by a moderator: May 5, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook