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

(i) Each spherical surface of a spherical lens forms a part of a sphere. The centres of these spheres are called the centres of curvature.
(ii) The principal axis of a lens is a imaginary line passing through its two centres of curvature.
(iii) The central point of a lens is called its optical centre. A ray of light passes undeviated through the optical centre of any lens.
(iv) The radii of two spherical surfaces of the lens are called radii of curvature.
(v) The point on the principal axis, at which incident light rays parallel to the principal axis converge or appear to diverge from, is called the principal focus of the lens.
(vi) The distance between the optical centre and the principal focus of a spherical lens is called its focal length.
(vii) The aperture of a spherical lens is the diameter of its circular outline.