Define the following terms in the context of spherical mirrors :
(i) Pole
(ii) Centre of curvature
(iii) Radius of curvature
(iv) Principal axis
(v) Principal focus

Explanation

Centre of curvature (C) : The centre of the spherws whose parts form the surfaces of a lens are called the centres of curvature of the lens. A lens has two centres of curvature `C_(1) and C_(2)` for its two sphercial surfaces.
Radii of curvature `(R_(1) , R_(2))` : The radii of the spheres whose parts form surfaces of a lens are called the radii of the curvature of the lens.
Principle axis : The imaginary straight line passing through the two centres of curvature of a lens is called principal axis of the lens.
Optical centre (O) : The point inside a lens on the principal axis, through which light rays pass without changing their path is called the optical centre (O) of the lens .
When light rays parallel to the principle axis are incident on a convex lens, they converage at a point on the principle axis. This point is called the principal focus (F) of the convex lens. Light rays travelling parallel to the principal axis of the concave lens diverage after refraction in such a way that they appear to teh coming out of a point on the principal axis. This point is called the principal focus of the concave lens. A lens has two principal foci `F_(1)` and `F_(2)`
The distance between the optical centre and the principal focus of a lens is called the focal length (F) of the lens.

The cross sections of convex and concave lenses are shwon in parts (a) and (b) fig.1. The surface marked as 1 is part of sphere `S_(1)` while the surface marked as 2 is part of sphere `S_(2)`.