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Index of refraction below one

by Robert_G
Tags: index, refraction
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Aug27-14, 07:15 AM
P: 36
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?
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Aug27-14, 07:47 AM
PF Gold
P: 377
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
Aug27-14, 07:53 AM
Sci Advisor
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P: 13,093
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:

But how can this be???

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