An erect image 2.0 cm high is formed 12 cm from a lens, the object being 0.5 cm high. Find the focal length of the lens.

Height of image = 2 cm

Image distance = 12 cm

Height of object= 0.5 cm

M = height of image/ height of object = 2/0.5 =- 4

M = v/u

-4 = 12/u

u =- 3 cm

using lens formula,

f = + 4.0 cm