Question

A concave mirror gives a real image magnified 4 times. When the object is moved 3 cm the magnification of the real image is 3 times. Find the focal length of mirror.

Answer 1

m = 4, v = 4u

when u’ = (u + 3), then m = 3

• 1/4u - 1/u = 1/f ⇒ f = 4u/5

-1/3(4 + 3) - 3/3(4 + 3) = 1/f ⇒ f = -3(4 + 3)/3

4u/5 = 3(4 + 3)/3 ⇒ 164 = 15(u + 3)

⇒ u = 45, Hence 1/-4 x 45 - 4/4 x 45 = 1/f

f = 36 cm