The distance PC is equal to the radius of curvature (R). It is crucial in understanding the geometry and properties of the spherical mirror, serving as a fundamental parameter in optical calculations and analyses.

The principal axis is a straight line passing through the pole and the center of curvature of a spherical mirror. It is normal to the mirror at its pole.

The radius of curvature (R) of a spherical mirror is the distance from the pole to the center of curvature. It is represented by the letter R.

For a concave mirror, the center of curvature lies in front of it, while for a convex mirror, it lies behind the mirror.

The center of curvature of a spherical mirror is the center of the sphere of which the reflecting surface forms a part. It is represented by the letter C.