# Definitions Related to Spherical Mirrors

## Pole:

The centre of the reflecting surface of the concave mirror is called its pole. The pole lies on the surface of the concave mirror and is denoted by P.

## Centre of Curvature:

Centre of curvature of the mirror is measured through the centre of the sphere in which the concave reflecting surface is a part. It lies external to the reflecting surface of the mirror and is denoted by C. For a concave mirror, the centre of curvature lies in front of the mirror.

It is the radius of the sphere which forms the reflective part. It is denoted by R.

## Principal Axis:

The principal axis is a straight line that passes through the pole and the centre of curvature of the mirror. It is always perpendicular to the pole.

## Principal Focus:

After reflection, the reflected rays which are parallel to the principal axis converge at a common point on the principal axis. This point is called the principal focus. It is denoted by F.

## Focal Length:

The distance between the pole and the principal focus is called the focal length. It is denoted by f.