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.
