An object is 10 m from a convex lens and an image forms on the other side of the lens, 20 m away. The focal length of the lens is
A) -20 m
B) 20 m
C) - 20/3 m
D) 20/3 m

Question

Quizplus · Accepted Answer

The lens formula is 1/f = 1/v - 1/u, where f is the focal length, v is the image distance (20 m, positive because the image is on the opposite side of the lens), and u is the object distance (-10 m, negative because the object is on the opposite side of the lens from the direction we consider positive). Plugging in the values, we get 1/f = 1/20 - (-1/10) = 1/20 + 1/10 = 3/20, so f = 20/3 m.

An Object Is 10 M from a Convex Lens and an Image