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Title: GR8677.072
Post by: conantee on January 06, 2010, 12:35:23 PM
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


Title: Re: GR8677.072
Post by: conantee on October 13, 2010, 08:34:39 AM
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 c 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.