Index of refraction below one

  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:

    “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. maajdl

    maajdl 374
    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. Simon Bridge

    Simon Bridge 15,471
    Science Advisor
    Homework Helper
    Gold Member

    Last edited: Aug 27, 2014
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