First, let\'s consider sound: (a) What is the critical angle in the case of the

Question

First, let's consider sound: (a) What is the critical angle in the case of the propagation of light for the interface between water (n = 1.33) and Lucite (n = 1.51)? (b) To be internally reflected, the light must start in which material? Now consider sound. In this case, let's define the index of refraction of a material for sound waves to be the ratio of the speed of sound in air to the speed of sound in the material. Snell's law then applies to the refraction of sound waves. Assume the speed of a sound wave is upsilon = 344 rn/s in air and upsilon = 1320 m/s in water. (a) Which medium has the higher index of refraction for sound? (b) What is the critical angle for a sound wave incident on the surface between air and water? (c) For total internal reflection to occur, must the sound wave be traveling in the air or in the water? (d) Use your results to explain why it is possible to hear people on the opposite shore of a river or small lake extremely clearly.

Explanation / Answer

a)We know the threshold condition for total internal reflection is:

ni sin(theta-c) = nt sin90

1.51 sin(theta-c) = 1.33

theta-c = sin-1 (1.33/1.51) = 61.74 deg

Hence, thets-c = 61.74 deg

b)To be internally reflected the light should start from Lucite.

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