Question Video: Drawing Rays Reflected from a Concave Mirror Science

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.