Why do x rays travel faster than light in some states of matter

[Gobil]

hi All,

why is this? some matter has a refractive index of slightly less than one for light in the x ray region. This implies a phase velocity faster than c, right? could someone explain what is actually happening here?

THANKS!

 

[DrDu]

The dielectric constant increases when approaching a resonance (normal dispersion), decreases rapidly in the resonance region (anomalous dispersion) and increases again after the resonance. In the optical region and below, the relative dielectric function is mostly >1, as the electonic resonances are somewhere in the UV. Above the UV all resonances are to the left and the dielectric function approaches 1 from below.
Phase velocity may take on (almost) any value you like, as signal velocity depends on group velocity, not on phase velocity.

 

[Gobil]

but normally the refractive index is described as c/v, where v is the velocity of the light in the medium with index n. so what is traveling faster than c in the case of xrays?

 

[Vanadium 50]

Can you point us at the originla source for what you write? Otherwise we are trying to explain something we can only guess about.

 

[Gobil]

well, it´s all from wiki

http://en.wikipedia.org/wiki/Refractive_index

which states that some wavelengths and materials have a refractive index less than 1

 

[DrDu]

but normally the refractive index is described as c/v, where v is the velocity of the light in the medium with index n. so what is traveling faster than c in the case of xrays?

The phase speed v in the medium is in deed higher than the phase speed in vacuo c.

 

[Vanadium 50]

And x-rays don't appear in the Wiki article. What exactly is your question?

 

[Gobil]

thanks Dr. Du,

does this mean the phase speed is greater than c in vacuo too, for some frequencies?

Vanadium 50,

From the posted wiki article:

"The number n is typically greater than one. However, at certain frequencies (e.g. near absorption resonances, and for X-rays), n will actually be smaller than one[7] (see also Cherenkov radiation). This does not contradict the theory of relativity, which holds that no information-carrying signal can ever propagate faster than c, because the phase speed is not the same as the group speed or the signal speed."