A mirror whose polished, reflecting surface is a part of a hollow sphere of glass or plastic is called a spherical mirror.
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 blue and the reflecting side is blue.
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 centre 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 centre of the sphere of which the mirror is a part.Example: Surface of a spoon curved inwards can be approximated to a concave mirror and the surface bulged outward can be considered as a convex mirror.
Let us now define certain physical terms relating to spherical mirrors.
Centre of Curvature
Centre of Curvature is the centre 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.
Pole
Pole is the midpoint of the aperture of the spherical mirror. It is represented by the letter P.
Principal Axis
Principal axis is the straight line passing through the pole and the centre of curvature of a spherical mirror.
Secondary Axis
Secondary axis is any other radial line passing through the centre of curvature other than the principal axis.
Normal
The normal at any point of the spherical mirror is the straight line obtained by joining that point with the centre of the mirror. The normal at point A on the mirror is the line AC obtained by joining A to the centre 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.
Principal Focus or Focus
The rays of light parallel to the principal axis of a mirror after reflection, either pass through a point (in case of a concave mirror) or appear to diverge from a point (in the case of a convex mirror) on the principal axis and this point is referred to as the principal focus or focal point of the mirror.
OR
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 Mirror 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 reflection 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. |
