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[conantee]

13. Two coherent sources of visible monochromatic light form an interference pattern on a screen. If the relative phase of the sources is varied from to at a frequecny of hertz, which of the following best describes the effect, if any, on the interference pattern?
(A) It is unaffected because the frequency of the phase change is very smalll compared to the frequency of visible light.
(B) It is unaffected because the frequency of the phase change is an integral multiple of
(C) It is destroyed except when the phase difference is or
(D) It is destroyed for all phase differences because the monochromaticity of the sources is destroyed.
(E) It is not destroyed but simply shifts positions at a rate too rapid to be detected by the eye.

[conantee]

Let's consider each choice and find what's wrong in each of them.
(A) is wrong since the frequency of phase change has nothing to do with the frequency of visible light. Since two sources are coherent, the frequency of visible light determines the distance between interference lines on a screen, while the phase difference between two sources is just off-set. They are irrelevant to compare.
(B) is wrong since the frequency of phase change is hertz, which is not intregral number of
(C) is wrong since the phase difference does not destroy pattern. The pattern is result of coherence.
(D) is wrong since monochromaticity of the sources is not destroyed. The phase change does not affect the frequency. Totally different matter.

Hence, the correct answer is (E). You can get this answer even though you are not positive about how the eye detects the frequency.
52 of 100 people got the right answer.