The measured index of refraction of X-rays in rock salt is less than one. This is consistent with the theory of relativity because
A. Relativity deals with light waves traveling in a vacuum only
B. X-rays cannot transmit signal
C. X-rays photons have imaginary mass
D. The theory of relativity predates the development of solid-state physics
E. The phase velocity and group velocity are different
(GR8677 #72)
Solution:
Index of refraction, n = c/v
For X-rays, n
1 → v 
c (violates the theory of special relativity)However, this is OK, because in this case, v is phase velocity, vp.
vp does not represent the particle velocity.
The physical speed of particle is represented by group velocity, vg.
Answer: E
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