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

Index of refraction below one

  1. Aug 27, 2014 #1
    Hi, there.

    I don't understand how the light travel in the medium which has the index of refaction below one

    from the following topic:
    http://en.wikipedia.org/wiki/Total_external_reflection

    “For X-rays, however, all materials have indices of refraction slightly below 1. ”

    I did some calculations before, for same light pulse, in certain materials, the index of fraction do have a value smaller than 1.

    Does this mean the light is travelling faster than c?
     
  2. jcsd
  3. Aug 27, 2014 #2

    maajdl

    User Avatar
    Gold Member

    Yes the phase velocity is larger than c.
    Yet, it is still impossible to transmit information or any "effect" faster than c.

    Read about: dispersion, group velocity, ...
    Materials are necessary dispersive because the interaction with light necessarily depends on the frequency (color).
     
  4. Aug 27, 2014 #3

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Last edited: Aug 27, 2014
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted