Video Transcript
Which of the following sentences is
a correct description of what will happen to parallel rays incident on a concave
mirror? (A) They will continue
undisturbed. (B) They will be focused at the
center of curvature. (C) They will be focused at the
focal point. (D) They will not be focused at a
point at all.
To see which of these answer
options is correct, let’s sketch in a concave mirror. And because this mirror is concave,
we know that incoming rays of light will approach it from this side. Now our question specifically talks
about parallel rays of incoming light. So let’s say we have two such rays
like this. Answer option (A) says that when
these rays reach the mirror, they will continue on undisturbed. If that happened, the path of the
rays would look like these dashed lines. We know though that one property of
a mirror is to reflect light. The incoming rays then won’t follow
the original paths they were on. We can eliminate answer option (A)
from consideration.
Options (B) and (C) talk about
these incoming rays being focused. That means when the rays are
reflected, they cross at a point. Answer option (B) says that this
focusing happens at a point called the center of curvature. Let’s now recall that our mirror is
actually part of a larger spherical surface. The center of that sphere, which is
right here, is at the point called the center of curvature. For our parallel incoming rays of
light, we see that this is not the point where the rays are brought to a focus. We can cross out answer option
(B).
Option (C) says that the rays are
focused at what’s called the focal point. This indeed is a correct
description of what happens to these incoming parallel rays. The point where the reflected rays
are brought to a focus is called the focal point. We see then that option (C) is
correct. And in the process, we can tell
that option (D) is not correct. We’ve seen that these parallel
incoming rays really are brought to a focus after being reflected. For our answer, we choose option
(C). When parallel rays of light are
incident on a concave mirror, they will be focused at the focal point.
