You know that reflection of light obeys the following laws:

Reflection of light

The angle of incidence Î is equal to the angle of reflection ř
(fig. 14.1)
The incident ray, the reflected ray and normal at the point of incidence, all lie in the same plane.
The reflection from spherical mirrors also obeys the above two laws.
Lesson 2. Spherical Mirrors
As we already know, a spherical mirror, in fact is a portion of the reflecting surface of a hollow sphere. Spherical mirrors are of two types:

1. Concave Mirror 2. Convex Mirror

Concave mirror is that whose inner curved surface is reflecting
Whereas a convex mirror is that shoes outer curved surface is reflecting (fig. 14.2-a,b).
convex-mirror

spherical-mirror

The centre C of the sphere, of which a concave mirror or spherical mirror and the “Centre of Curvature” of the spherical mirror and the “Radius of Curvature” R of the mirror is the radius of this sphere. The front section of spherical mirror is circular one and its diameter is known as the “Aperture”.

Principal Axis

principal axis

The centre P of the mirror is called the pole of the mirror. A line joining the pole P of the mirror C, the centre of curvature, is called the “Principle Axis” of the mirror.

The principal focus

Rays of light parallel to the principal axis reflection from a concave mirror converge to a point F. this point is called the “principal focus” of the mirror (fig. 14.3-a). Since rays, in fact, pass through this point, therefore, it is called real focus. In the case of a convex mirror, rays parallel to the principal axis after reflection appear to come from a point F situated behind the mirror. This point is called the principal focus of the convex mirror. The principal focus of a convex mirror is a virtual focus because the reflected rays do not actually pass through it, but appear to do so (fig. 14.3-b) therefore, its focus is called virtual focus.

principal-focus

The distance between the pole and the principal focus of a spherical mirror (concave as well as convex ) is called the “focal length “.it is denoted by f .the radius of curvature of a spherical mirror is twice of its focal length i.e.,

f=R/2