# The radius of curvature of a concave miror is 20cm. Its focal length is what?

## Question:

The radius of curvature of a concave miror is 20cm. Its focal length is what?

## Spherical Mirrors

A spherical mirror is cut from a sphere. That means that it has a center of curvature. This is the center of the sphere from which it was cut. There is also the radius of curvature that corresponds to this center. The focal length of a spherical mirror is always halfway between the center and focus. This means that the focal length is half the radius of curvature.

{eq}f=\frac{R}{2} {/eq}

A concave mirror is a spherical mirror. As a result, its focal length is half the radius of curvature of the mirror.

{eq}f=\frac{R}{2}\\ f=\frac{20 \ cm}{2}\\ f=10 \ cm {/eq}