An object placed at 20 cm in front of a concave mirror produces three times magnified real image. What is the focal length of the concave mirror?

Options

(a) 15 cm
(b) 6.6 cm
(c) 10 cm
(d) 7.5 cm

Correct Answer:

15 cm

Explanation:

We know that, linear magnification, m = f / (f -u) —-(i)

Given, object displaced, u = -20 cm

m = -3 (All real images are inverted)

Substituting the above values in equation (i), we get

-3 = f / [f – (-20)]

⇒ -3f – 60 = f 4f = -60

⇒ f = – 60 / 4 = -15 cm