The focal length of a spherical mirror is equal to:

The correct answer is Option (2) → Half of its radius of curvature

The radius of curvature (R) of a spherical mirror is the radius of the sphere of which the mirror is a part; its centre is the centre of curvature C. The focal length (f) is the distance from the pole of the mirror to its principal focus F, the point where rays parallel to the principal axis meet (or seem to come from) after reflection.

For rays close to the principal axis, the principal focus lies midway between the pole and the centre of curvature, so

• \(\displaystyle f=\frac{R}{2}\)

This holds for both concave and convex mirrors; a mirror with radius of curvature 30 cm, for example, has a focal length of 15 cm.