Explanation:

When light rays fall on a spherical mirror, they either converge or diverge depending upon the type of mirror. The radius of curvature of a mirror is the distance between the center of curvature and the vertex of the mirror. The focal length of a mirror is the distance between the center of curvature and the focal point of the mirror.

Formula:

The formula to find the focal length of a spherical mirror is:

1/f = 1/u + 1/v

Where,
f = focal length of the mirror
u = distance of the object from the mirror
v = distance of the image from the mirror

Given:

Radius of curvature (R) = 20 cm

Using Formula:

The relation between radius of curvature and focal length is:

f = R/2

Substituting the given value of radius of curvature in the above formula, we get:

f = 20/2
f = 10 cm

Therefore, the focal length of the mirror is 10 cm.

Conclusion:

The focal length of a spherical mirror can be found using the formula 1/f = 1/u + 1/v, where u is the distance of the object from the mirror and v is the distance of the image from the mirror. However, if the radius of curvature is given, we can use the formula f = R/2 to find the focal length. In this case, the radius of curvature was 20 cm, so the focal length of the mirror was found to be 10 cm.