When an object is placed normally on the principal axis of a spherial mirror at a distance 'a' from its pole, its image is formed at a distance 'v' from the pole of mirror such that `(1)/(v)+(1)/(u)=(1)/(f)`, where f = focal length of given mirror.
The relation is called mirror formula and is true for all types of mirrors under all conditions. However, values of u, v and f must be put with proper signs are per the sign convention followed. If 'h' be the height of the linear object and h' the height of image, then ratio `(h')/(h)` is called linear magnification or lateral magnification and its value m is given as :
`m=(h')/(h)=-(v)/(u)`
(a) A planer object of length 3 cm and breadth 2 cm is held normal to the principal axis of a concave mirror of focal length 20 cm at a distance of 30 cm from its pole. Where is its image formed?

Explanation

(a) Here `u=-30cm, f=-20cm` and area of planar object `A=lb=3xx2=6cm^(2)`.
`therefore" "(1)/(v)=(1)/(f)-(1)/(u)=(1)/((-20))-(1)/((-30))=-(1)/(60)`
`rArr" "v=-60cm`
thus, image is formed in front of mirror at a distance of 60 cm from the pole.