The measured index of refraction of x-rays in rock salt is less than one. this is consistent with the theory of relativity because Vp is phase velocity. Vp does not represent the particle velocity. The physical speed of the particle is represented by group velocity, Vg.
The refractive index of an optical medium is a dimensionless number that indicates how well that medium bends light in optics. The refractive index controls how much light is refracted or twisted when it enters a substance.
Index of refraction, n = c/v
For X-rays, n is 1 → v > c (violates the theory of special relativity) However, this is ok because in this case, Vp is phase velocity. Vp does not represent the particle velocity. The physical speed of the particle is represented by group velocity, Vg.
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