In a spherical mirror, one of the two curved surfaces is coated with a thin layer of silver followed by a coating of red lead oxide paint. Thus, one side of the spherical mirror is opaque and the other side is a highly polished reflecting surface. In a diagram the opaque side of a mirror is always shown shaded.
In the diagrams given here, please remember that the opaque, non reflecting side is shaded grey. The reflecting side is black.Depending upon the nature of the reflecting surface of a mirror, the spherical mirror is classified as:
- Concave mirror
- Convex mirror
Concave Mirror
Concave mirror is a spherical mirror whose reflecting surface is towards the center of the sphere of which the mirror is a part.
Convex Mirror
Convex mirror is a spherical mirror whose reflecting surface is away from the center of the sphere of which the mirror is a part.
Concave and Convex Mirror
Let us now define certain physical terms relating to spherical mirrors.Center of Curvature
Center of Curvature is the center of the sphere of which the spherical mirror forms a part. It is denoted by the letter C.
Radius of Curvature
Radius of Curvature is the radius of the sphere of which the mirror is a part. It is represented by the letter R.
Linear Aperture
Linear aperture is the distance between the extreme points (X and Y) on the periphery of the mirror.
XY is the Aperture
Pole
Pole is the midpoint of the aperture of the spherical mirror. It is represented by the letter P.
Midpoint of xy
Principal Axis
Principal axis is the straight line passing through the pole and the center of curvature of a spherical mirror.
Principal Axis
Secondary Axis
Secondary axis is any other radial line passing through the center of curvature other than the principal axis.
Secondary Axis
Normal
The normal at any point of the spherical mirror is the straight line obtained by joining that point with the center of the mirror. The normal at point A on the mirror is the line AC obtained by joining A to the center of curvature of the mirror. Normal at any point on a spherical mirror is equal to the radius of the sphere of which the mirror is a part.
Normal
Principal Focus or Focus
The principal focus of a spherical mirror may be defined as a point on its principal axis where a beam of light parallel to the principal axis converges to or appears to diverge from after reflection from the spherical mirror.
Focal Length
Focal length is the distance between the pole and the focus of a mirror. It is represented by the letter f.Characteristics of Focus of a Concave and a Convex Mirror
| Convex Mirror | Concave Mirror |
|---|---|
| The focus lies behind the mirror | The focus is in front of the mirror |
| The focus is virtual as the rays of light after reflection appear to come from the focus | The focus is real as the rays of light after reflection converge at the focus |
Note : A concave mirror is also known as a converging mirror as the parallel rays of light after getting reflected from the concave mirror converge at the focus.
A convex mirror is known as a diverging mirror as the parallel rays of light after reflection appear to come from a point, i.e., the rays diverge after reflection.
