Why does n<1 for X-rays suggest faster than light?

[manesh]

Faster than c!

hi
I read that the refractive index(n) for X-rays in meterials are less than 1(very little less than 1). I got confused ..I am using X-ray reflection at the moment. But I am not convinced by the fact that n<1 means faster than c(velocity of light in vacuum or in air)!
hope somebody can explain the contradiction
cheers
Manesh

 

[EIRE2003]

I looked this up over the net and ka-boom! the refractive index of x-rays is slightly less than 1. So could this mean that when x-rays undergo refraction they travel faster than 3x10^8?

What material is this by the way that refractive index is slightly less than one?

Oh and I just thought that I would bring this up, c can also be slowed to 38mph when passed through the 6th state of matter - bose einstein condensates.

 

[nrqed]

I looked this up over the net and ka-boom! the refractive index of x-rays is slightly less than 1. So could this mean that when x-rays undergo refraction they travel faster than 3x10^8?

What material is this by the way that refractive index is slightly less than one?

Oh and I just thought that I would bring this up, c can also be slowed to 38mph when passed through the 6th state of matter - bose einstein condensates.

The index of refraction is related to the phase velocity which, in some sense, is not a physically meaningful concept (it would require a wave of definite wavelength, hence of infinite extent). What matters for the propagation of wavepackets is the group velocity, which is probably less than c (I say probably because there are exceptions to that statement too, in regions of anomaloous dispersion, in which case one must introduce a so-called "signal velocity" )

Pat

 

[da_willem]

https://www.physicsforums.com/showthread.php?t=52570