Question

If focal length of a mirror is $ + 15cm$, then it is
(A) Convex mirror
(B) Concave mirror
(C) Plane mirror
(D) None of these

Answer 1

Hint When the rays of light coming parallel to principal axis are incident on a reflecting surface, after reflecting all the rays are either converge to a point or appear to converge at a point and the point where they converge is focus point of reflection surface. The distance between the reflecting surface and focus point is known as focal length.

Complete step by step solution
We know for a plane reflect rays are also parallel they never converge at a point but they appear to be converged at infinity means focal length of plane mirror is infinity.
As we use, distance in the direction of incident rays is negative and measures from the mirror and on the other side are positive.
In the case of convex mirrors, focal length is positive because the focus point is on the other side than the incident rays coming. But for a concave mirror, focal length is negative as the focus point is on the same side from which incident rays are coming.
Then there is only one option for which focal length is $ + 15cm$ that is a convex mirror.

Hence the correct answer is option A.

Note If an object is placed at infinity of a mirror then its image is formed at the focus point of the mirror because rays coming from infinite are always parallel to the principal axis. And if object is placed at focus point then, its image is formed at infinity. The image formed at infinity is very large in size.