SAT Subject Physics Practice Question 589: Answer and Explanation

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Question: 589

15. Consider a light wave that passes from air into a very thick clear glass block that has its opposite internal side mirrored (facing into the glass). The light ray passes into the glass block at an angle greater than 0° and less than 90°, strikes the mirrored surface and reflects back through the glass into the air.

What happens when the light ray leaves the glass block after it has struck the mirrored surface?

A. The waves reflect back on the glass.
B. The waves increase velocity.
C. The waves leave the glass at the same angle at which they entered the glass.
D. The angle of refraction is greater than the angle of incidence.
E. The angle of reflection from the mirror is equal to the incident angle at which the light struck the glass block.

Correct Answer: C


The correct answer is (C). The light ray is refracted toward the normal passes through the glass and is incident on the mirrored surface. The angle of incidence equals the angle of reflection, and the light ray reflects toward the same surface it entered. At the surface the light ray refracts away from the normal, producing a larger angle than the approach (incident) angle. The angle at which the ray leaves the glass block is the same angle at which it originally approached the glass block.

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