![](https://i.imgur.com/aCXx6mW.jpg)
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.
![](https://i.imgur.com/aCXx6mW.jpg)
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.
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.