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.
The Radius of Curvature:
It is the radius of the sphere which forms the reflective part. It is denoted by R.
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.
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.
The distance between the pole and the principal focus is called the focal length. It is denoted by f.