A lens of focal length 10 cm is placed in contact with a mirror. What is the effective focal length of the combination?
Correct: B
When a lens is placed in contact with a mirror, the effective focal length is given by 1/f = 1/f1 + 1/f2, where f1 is the focal length of the lens and f2 is the focal length of the mirror. Since the focal length of a mirror is equal to half its radius of curvature, which is infinity for a plane mirror, 1/f2 = 0. So 1/f = 1/f1 = 1/10, which gives f = 10 cm.