**Definitions Related to Spherical Mirrors:**

**Pole:** The central point of the reflecting surface of a spherical mirror is called the pole of the mirror. In figure (1), P is the pole of the mirror. Pole is the lowest point of the reflecting surface in case of a concave mirror, while it is the highest point in a convex mirror.

**Center of curvature: **The centre of the sphere of which the spherical minor is a part is called the centre of curvature of that minor. In figure (1), C is the centre of curvature.

Radius of curvature: The radius of the sphere of which the spherical minor is a part is called the radius of curvature of that mirror. In figure (1), PC or MC is the radius of curvature. Radius of curvature is denoted by r.

**Principal axis:** The straight line passing through the pole and the centre of curvature of a spherical mirror is called the principal axis of that mirror. In figure (1), PC is the principal axis.

**Secondary axis: **The straight line passing through the centre of curvature and any point other than the pole on the reflecting surface of the sphere is called the secondary axis. In figure (1), P’C is the secondary axis.

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