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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:

    “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


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    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

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    Last edited: Aug 27, 2014
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