The focal length of a spherical mirror is equal to:

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

For a spherical mirror (concave or convex), the focal length f is related to the radius of curvature R (distance from the pole to the center of curvature) by the formula:

$f = \frac{R}{2}$​

This is derived from the mirror formula and geometry: the focus is the midpoint between the pole and the center of curvature for paraxial rays.

• Sign convention: f is negative for convex mirrors and positive for concave; R follows the same sign.
• Magnitude: $|f| = \frac{|R|}{2}$

Thus, the focal length is half the radius of curvature.