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Light through difrent substances

  1. Sep 22, 2007 #1
    Does light travel difrent speeds through, difrent substances. I know it would always be traveling the same speed but would it apear to be traveling a difrent speed from bouncing off anything... or well im not sure thats why im asking lol. I heared about some kind of experiment where you shine a laser through some type of crystal and the light apears to exit the material before it enters :S
    Very confused
  2. jcsd
  3. Sep 22, 2007 #2
    for example when light hits water it bends this must in some way effect how fast it seems to be moving to us, does it keep bending as it goes through or just travel a straight line.
    Or when light hits earths atmosphere from space does it bend like it would from air into water and if so does it continue to hit particles as it travels or just travel a straight line?
  4. Sep 22, 2007 #3


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    Light changes speed in different materials always slower than compared to vaccum.
    In air it goes about 1.003 times slower, in diamond 2.4 times.
    Actually a better way to think is that a photon travels at the speed of light between atoms, but when it hits an atom it is absorbed and emitted a very short time later. It is this delay which makes it look like the light is going slower.

    The bending effect only happens at a change of material (an interface) in the material it goes in a straight line. When light enters the atmosphere from space it bends through a small angle like when a straw in a glass of water appears bent. You have to take this into account when measuring the position of stars.
  5. Sep 23, 2007 #4

    Claude Bile

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    I think some confusion here is due to the misconception that light always travels at c. This is not so. When light travels through media, be it gas liquid or solid, the media will (among other things) result in the group velocity of the wave being reduced.

    The experiment you refer to is an application of the quantum properties of the photon, quantum entanglement or tunneling I believe, I can't recall exactly which.

  6. Sep 23, 2007 #5


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    Please read the FAQ in this sub-forum.

  7. Sep 23, 2007 #6


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    To be more correct it is not absorbed by the atoms but by bonds in the material - I oversimplified.
  8. Sep 24, 2007 #7
    There have been several experiments where superluminal propogation APPEARS to take place. In all of these experiments, one needs to know what group, phase, and signal velocities are in order to understand why no violation of SR is going on. I think the specific example you gave about superluminal velocity in a crystal was done in 2002 by a Canadian team that sent pulses through a coaxial photonic crystal [1]. If you've figured out by now that the "group velocity" of the pulse was what was going superlumnial, you'd be correct. Maybe you are thinking of another experiment?

    [1] A. Haché and L. Poirier, Appl. Phys. Lett. 80 (2002), p. 518
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