Question

Explain principal focus of mirrors with the help of suitable diagrams.

Answer 1

(a) A number of rays parallel to the principal axis after reflection from a concave mirror meet at a point on the principal axis or appear to come from a point after reflection from a convex mirror on the principal axis. This is called principal focus.

(b) When the object is placed beyond C.

© f = - 12 cm, u = - 6 cm, v = ?

Using the expression

1/f = 1/v - 1/u , we have

1/-12 = 1/v - 1/-6

v = 12 cm

Answer 2

In concave mirrors ::-

The rays parallel to principal axis meet at one point on the principal axis , after reflection.This point is known as principal focus (f).

In convex mirrors ::-

The rays parallel to principal axis diverge after reflection.But, when extended backwards , they appear to meet at one point on the principal axis. This point is known as principal axis.

Refer to following diagrams ::-