When an object is 12 cm in front of a concave spherical mirror, the image is formed 3.0 cm in front of the mirror. What is the focal length of the mirror?
A) 15 cm
B) 2.4 cm
C) 1.3 cm
D) 4.0 cm

Question

Quizplus · Accepted Answer

The mirror equation is 1/f = 1/v + 1/u, where f is the focal length, v is the image distance, and u is the object distance. Given v = -3.0 cm (in front of the mirror is considered negative in mirror equation convention) and u = -12 cm (object in front of the mirror is also considered negative), we can solve for f: 1/f = 1/(-3) + 1/(-12) = -1/4, so f = -2.4 cm. The negative sign indicates that the focal length is on the same side as the object, which is characteristic of a concave mirror.

When an Object Is 12 Cm in Front of a Concave