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[conantee]

72. 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 signals
(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

[conantee]

You can do this by eliminating choices.
(B) is obviously wrong. x-ray can transmitted signals - like other photons (radio wave, visible light etc.)
(D) is also wrong. The theory of relativity can be applied to solid-state physics
(C) is wrong. This requires a bit of knowledge in relativity. Photons actually has zero rest mass - not imaginary mass.
(A) is wrong. Relativity applies to in other medium too, just like Newtonian mechanics or E&M. There are several questions in relativity class on light in material with n>1.

The correct answer is (E). The phase velocity is the velocity of phase. It can be greater than since it does not represent the speed of physical object, while the group velocity does.

The correct answer is (E). 63 of 100 people answered this question correctly.