Is Light Traveling Faster Than c in Materials with Index of Refraction Below 1?

[Robert_G]

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 traveling faster than c?

 

[maajdl]

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).

 

[Simon Bridge]

It is possible for a light pulse to travel faster than c in a medium - it's an interference effect.
The phase velocity of x-rays is routinely faster than c.

This gets discussed here a fair bit:
https://www.physicsforums.com/showthread.php?t=238919

But how can this be?
http://www.mathpages.com/home/kmath210/kmath210.htm