Video Transcript
The following figure shows three
light rays incident on a concave mirror. Which is the correctly drawn
reflected ray, ray A or ray B? (A) Ray A or (B) ray B.
In this question, we have three
incident rays, each at a different angle. We can see that all of the incident
rays pass through a single point. And we can see that the green and
blue reflected rays are parallel with each other.
Recall that the focal point of a
concave mirror is the point where reflected rays of light that originated from
incident rays of light parallel to the principal axis of the mirror will cross. And importantly, this also works
the other way around. Reflected rays parallel to the
principal axis of a concave mirror have incident rays that pass through the focal
point.
In this case, the principal axis of
the mirror is in line with the green incident ray. We can see that the green and blue
reflected rays are parallel to this principal axis. So, the point where the green and
blue incident rays cross must be the focal point of the mirror. That’s this point here. We can see that the red incident
ray also passes through the focal point of the mirror. So its reflected ray must be
parallel to the principal axis of the mirror as well. This corresponds to ray A.
So the correct answer to this
question is option (A), ray A.
