Spherical refracting surface: A spherical refracting surface is formed when the refracting surface forms a part of a sphere. It is of two types:
(i) Convex spherical surface: If the convex surface is towards the rarer medium, it is called convex spherical refracting surface, Fig. (a).
(ii) Concave spherical surface: If the concave 1 surface is towards the rarer medium, it is called concave spherical refracting surface, Fig. (b).
1. Pole (P): The centre of spherical refracting 1 surface is called its pole.
2. Centre of curvature (C): It is the centre of the | sphere from which the curved surface is taken a part.
3. Radius of curvature (PC): It is the radius of the sphere from which the curved surface is taken a part.
4. Aperture (XY): The diameter of the curved surface is called its aperture.
5. Principal axis: The principal axis is the line joining the pole and centre of curvature of refracting surface.
