The relationship between radius of curvature (R) and focal length (f) of a spherical mirror is

CONCEPT:

• Spherical mirrors are a part of the whole sphere.

• Focal length (f): The distance between the centre of the mirror and the focal point (where parallel rays of light meet or appear to meet) of the mirror is called the focal length.
• The radius of curvature (R): The distance between the centre of the lens or mirror to the vertex which is located on the local optical axis is called the Radius of Curvature.
• The focal length is half of the radius of curvature.

f = R/2

EXPLANATION:

The relationship between the radius of curvature and focal length is given by:

R = 2f 

The radius of curvature is twice the focal length. So option 3 is correct.