A concave mirror of radius of curvature 40 cm forms a real image twice as large as the object. The object distance is?

A. 10 cm
B. 30 cm
C. 40 cm
D. 60 cm

Correct Answer:

Option B – 30 cm

Explanation

if r = 40 cm , then f = r/2 = 40/2 = 20 cm
then for a real image by a concave mirror,
magnification M = f/u-f => 2 = 20/U-20
therefore 2(u-20) = 20
2u – 40 = 20
2u = 40 + 20
2u = 60/2 = 30 cm