IMPORTANT TERMS RELATED WITH SPHRICAL MIRRORS
Centre of Curvature of Spherical Mirros
The centre of curvature of a spherical mirror is the centre of the hollow glass sphere of which the mirror is a part. It is represented by the letter ‘C’. In fig.(a) and fig.(b) C is the centre of curvature of concave mirror and convex mirror respectively. Here one thing worth to be noted is that the centre of curvature of a concave mirror is always in front of it while the centre of curvature of a convex mirror is always behind it.
Pole of Spherical Mirrors
Pole is the middle point or centre of a spherical mirror. It is represented by letter ‘P’.
Radius of Curvature of Spherical Mirrors
The radius of curvature of a spherical mirror is the radius of the hollow glass sphere of which the mirror is a part. It is represented by letter ‘R’. So PC = R.
Aperture of Mirror
The actual size MM’ of a spherical mirror is called the aperture of the mirror. It may be defined as that portion of a spherical mirror from which the reflection of light is actually taking place.
Principal Axis of Spherical Mirrors
The straight line passing through the pole and centre of curvature of a spherical mirror and extending on both sides is called principal axis of the spherical mirror.
Principal Focus and Focal Length of a Spherical Mirror
Principal Focus of Spherical Mirrors
The principal focus of a spherical mirror is a point on the principal axis of the spherical mirror at which, the light rays which are parallel to the principal axis actually converge (meet) or appear to diverge after reflection. The principal focus of a spherical mirror is represented by letter F.
Principal Focus of the Concave Mirror
In the case of concave mirror all the light rays which are parallel to the principal axis meet at point F after reflection. So, the principal focus of a concave mirror is a real focus.
Principal Focus of the Convex Mirror
In the case of convex mirror, all the light rays which are parallel to the principal axis appear to diverge after reflection as shown in figure (b1). Thus, we extend all the reflected rays in backward direction by dotted lines, so that they appear to meet at point F. this point is the principal focus of a convex mirror. It is a virtual focus because it is obtained by extending the light rays in backward direction.
Focal Length of Spherical Mirrors
The focal length of a spherical mirror is the distance of principal focus from the pole of the mirror. Thus in figure (a1) and (b1), the distance PF is the focal length of a spherical mirror. The focal length of a spherical mirror is represented by letter ‘f’. So PF = f.
Test Your Understanding and Answer These Questions:
- Define center of curvature of a spherical mirror.
- Define radius of curvature of a spherical mirror.
- Define pole and principal axis of a spherical mirror.
- Define principal focus and focal length of a spherical mirror by drawing well labeled diagram.
- Give the relationship between focal length and radius of curvature of a spherical mirror.