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.