Question

The focal length of concave mirror is 20 cm. where should an object be placed to get an image magnified twice?

Answer 1

Since it is 2 times magnified, m = -2

so -v/u = -2

similarly, 1/v + 1/u = 1/f

so 1/v + 1/u = 1/-20

-v/v + -v/u = -v/-20 (multiply throughout by -v)

-1 -2 = v/20

-3 = v/20

v = -60

-2 = -v/u

-2 = 60/u

u = -30

so the object must be placed 30 cm from the mirror.