Chapter-9|Ray Optics and Optical Instrument|NCERT 12th Physics:
A spherical mirror is a reflecting surface which forms part of a hollow sphere.
(I) Concave Mirror
(II) Convex Mirror
(I) Concave Spherical Mirror: A spherical mirror whose reflecting surface is towards the centre or towards the inner hollow sphere is called concave spherical mirror.
(II) Convex Spherical Mirror: A Spherical mirror whose reflecting surface is away from the centre or outer bulged surface is called convex spherical mirror.
To derive the relevant formulae for reflection by spherical mirrors and refraction by spherical lenses, we must first adopt a sign convention for measuring distances.
(I) All ray diagrams are drawn with the incident light traveling from left to right.
(II) All distances are measured from the pole of the mirror.
(III) All distances measured in the direction of incident light are taken to be positive.
(IV) All distances measured in the positive direction of incident light are taken to be negative.
(V) Heights measured upwards and perpendicular to the principal Axis are taken positive.
(VI) Height measured downwards and perpendicular to the principal axis are taken negative.