Principle focus of a concave mirror is a point on the principal axis at which a beam of light incident parallel to the principal axis converges after reflection from the concave mirror.
Focal length of a spherical mirror = 1/2 × radius of curvature
= \(\frac{1}{2}\) x 20 = 10 cm
∴ f = 10 cm
Concave mirror gives an erect and enlarged image of an object.
