Question

A concave mirror of focal length 20 cm forms an virtual image having twice the linear dimensions of the object, the position of the object will be __________ cm 

(A) 7.5 (B) – 10 (C) 10 (D) – 7.5

Answer 1

The correct option is (C) 10.

Explanation:

Focal length of concave mirror = f = 20 cm

magnification = m = 2 

m = (–v / u) 

∴ 2 = (–v / u) 

Hence v = –2u .....(1) 

Also (1/f) = (1/v) + (1/u) 

∴ (1/f) = (1 / –2u) + (1/u) 

(1/f) = {(– 1 + 2) / (2u)} = (1 / 2u) 

∴ 20 = 2u 

u = 10 cm i.e. position of object is 10 cm.