Question

The focal length of a concave mirror is nearly _________ of its radius of curvature.

Answer 1

1. Radius of curvature: The radius of the sphere of which the mirror is a part is called the radius of curvature of the mirror. It is thus the distance between the pole and the centre of curvature of the mirror.
2. Focal length: The focal length of a mirror is the distance between its pole and the principal focus.
3. The radius of curvature ($R$) of a spherical mirror is nearly twice its focal length (f).
4. Thus, the relation between focal length and radius of curvature is given by, $R=2f$ or $f=\frac{R}{2}$
5. This relation is true for convex as well as the concave mirrors

Thus, the focal length of a concave mirror is nearly half of its radius of curvature.