The reflecting surface of a spherical mirror may be curved inwards or outwards.
Spherical mirrors are of two types
1. Concave mirror: - In a concave mirror reflection of light takes place at the concave surface or bent-in surface as shown below in the figure.
2. Convex mirror:- In a convex mirror reflection of light takes place at the convex surface or bent out surface as shown below in the figure
Commonly used terms about Spherical mirrors :-
Centre of curvature: - The reflecting surface of a spherical mirror forms a part of a sphere. This sphere has a centre. This point is called the centre of curvature of the spherical mirror. It is represented by the letter C. Please note that the centre of curvature is not a part of the mirror. It lies outside its reflecting surface. The centre of curvature of a concave mirror lies in front of it. However, it lies behind the mirror in case of a convex mirror as shown above in the figure 2.
Radius of curvature: - The radius of the sphere of which the reflecting surface of a spherical mirror forms a part, is called the radius of curvature of the mirror. It is represented by the letter R.
Pole: - The center of a spherical mirror is called its pole and is represented by letter P as can be seen in figure 2.
Principle axis: - Straight line passing through the pole and the centre of curvature of a spherical mirror is called principle axis of the mirror.
Aperture of the mirror: - Portion of the mirror from which reflection of light actually takes place is called the aperture of the mirror. Aperture of the mirror actually represents the size of the mirror.
Principle focus and focal length of a Spherical Mirrors
For understanding about principle focus and focus length of a spherical mirror first consider the figure given below
From figure 3a we see that a number of rays parallel to the principal axis are falling on a concave mirror. If we now observe the reflected rays we see that they are all intersecting at a point
F on the principal axis of the mirror. This point is called the principal focus of the concave mirror.
In case of convex mirror rays get reflected at the reflecting surface of the mirror and these reflected rays appear to come from point F on the principle axis and this point F is called principle focus of convex mirror.
The distance between the pole and the principal focus of a spherical mirror is called the focal length. It is represented by the letter f.
There is a relationship between the radius of curvature R, and focal length f, of a spherical mirror and is given by R=2f which means that that the principal focus of a spherical mirror lies midway between the pole and centre of curvature.