SAT Subject Physics Practice Question 123: Answer and Explanation

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Question: 123

6. If a clear liquid has a refractive index of 1.45 and a transparent solid has an index of 2.90 then, for total internal reflection to occur at the interface between these two media, which of the following must be true?

incident beam originates in     at an angle of incidence greater than

A. (A) the solid          30°
B. (B) the liquid          30°
C. (C) the solid          60°
D. (D) the liquid          60°
E. (E) Total internal reflection cannot occur.

Correct Answer: A


The critical angle for total internal reflection is computed as follows:

Total internal reflection can happen only if the incident beam originates in the medium with the higher index of refraction and strikes the interface of the other medium at an angle of incidence greater than the critical angle.

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