A small insect viewed through a convex lens is 12 cm from th

A small insect viewed through a convex lens is 1.2 cm from the lens and appear twice its actual size. What is the focal length of the lens?

Solution

m = hi/ho = -di /do

2 = - di / do

and object distance is given , do = 1.2 cm

di = - 2 x 1.2 = - 2.4 cm

Applying lens equation,

1/f = 1/di + 1/do

1/f = 1/(-2.4) + 1/1.2

f = 2.4 cm ......Ans