MCAT Physics Question 114: Answer and Explanation

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Question: 114

9. A submarine is inspecting the surface of the water with a laser that points from the submarine to the surface of the water and through the air. At what angle will the laser not penetrate the surface of the water but rather reflect entirely back into the water? (Assume n_water = 1.33 and n_air = 1.)

A. 19°
B. 29°
C. 39°
D. 49°

Correct Answer: D

Explanation:

This question is testing our understanding of total internal reflection. As the laser beam travels from water to air—that is, from a higher to a lower index of refraction—the angle of refraction increases. At the critical angle (θ_c), the angle of refraction becomes 90°; at this point, the refracted ray is parallel to the surface of the water. When the angle of incidence is greater than the critical angle, all the light is reflected back into the water. The question is asking for the critical angle:

The inverse sine of 0.75 must be slightly higher than 48.59° is the exact answer.

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