Question

A concave mirror of focal length 20 cm forms an image having twice the
size of object. For the virtual position of object, the position of object
will be at
(a) 25 cm
(b) 40 cm
(c) 10 cm
(d) At infinity

Answer 1

Answer:

25 cm

Explanation:

Answer 2

Answer:

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.