Question Video: Describing the Behavior of Parallel Rays Incident on a Convex Mirror Science

Which one of the following sentences is the correct description for what happens to parallel rays incident on a convex mirror? [A] They will continue undisturbed. [B] They will be focused at a point, which is called the focal point. [C] They will not be focused at a point, but the mirror will still have a focal point. [D] They will not be focused at a point and the mirror will have no focal point.

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Video Transcript

Which one of the following sentences is the correct description for what happens to parallel rays incident on a convex mirror? (A) They will continue undisturbed. (B) They will be focused at a point which is called the focal point. (C) They will not be focused at a point, but the mirror will still have a focal point. (D) They will not be focused at a point, and the mirror will have no focal point.

To see which answer is correct, let’s clear some space at the top of our screen. And we can draw parallel rays incident on a convex mirror. Because these rays are reaching a mirror, they will reflect off of it. The center ray will bounce straight backward in this direction, while the top and bottom rays will reflect like this.

Looking again at our answer choices, we see that answer (A) can’t be correct. These rays do not continue undisturbed; instead, they reflect off the mirror. Option (B) says that the rays are focused to a point, but we see that these reflected rays travel farther and farther away from one another over time. Choice (B) can’t be correct either. To figure out which of options (C) and (D) is correct, we need to know whether this mirror has a focal point. To find out, let’s trace the reflected rays backward using dashed lines.

Tracing the center reflected right back would give this line. And then the top and bottom reflected rays would be traced backward like this. These rays do cross, which means there is a focal point. This means choice (C) is our answer. The reflected rays will not be focused at a point, but the mirror will still have a focal point. It’s the point we’ve identified in orange.

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