Define the radius of curvature of a spherical mirror.
If the radius of curvature of a spherical mirror is 20 cm, what is its focal length?

The radius of the curvature of the mirror is defined by the radius of the sphere from which the mirror was cut from.
Here,
Radius of curvature (R) = 20 cm
We know that,
Radius of curvature of a spherical mirror, R = 2 x Focal length (f)

$R=2f$

$f=\frac{R}{2}$

$f=\frac{20}{2}$

$f=10cm$

Hence, the focal length of the given spherical mirror is 10 cm