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

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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!
 

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  • #2
DrDu
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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.
 
  • #3
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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?
 
  • #4
Vanadium 50
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Can you point us at the originla source for what you write? Otherwise we are trying to explain something we can only guess about.
 
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DrDu
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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.
 
  • #7
Vanadium 50
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And x-rays don't appear in the Wiki article. What exactly is your question?
 
  • #8
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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."
 

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