When an object of height 1 cm is kept at a distance 4 cm from a concave mirror, it’s erect image of height 1.5 cm is formed at a distance 6 cm behind the mirror. Find the focal length of the mirror.

u = 4 cm (negative)

v = 6 cm (positive)

1\over u + 1\over v = 1\over f

⇒ – 1\over 4 + 1\over 6 = 1\over f

⇒ 1\over f = -3 + 2\over 12

⇒ 1\over f = -1\over 12

⇒ f = -12 cm

Hence, the focal length of concave mirror = 12 cm