A mirror has a focal length −20 cm. If an object is placed 10 cm in front of the mirror, where will the image form?
A) 20 cm in front of the mirror
B) 20 cm behind the mirror
C) 6.7 in front of the mirror
D) 6.7 behind the mirror
E) no image will form

Question

Quizplus · Accepted Answer

The mirror is concave since the focal length is negative. Using the mirror equation, 1/f = 1/do + 1/di, where f is the focal length, do is the object distance, and di is the image distance, we can solve for di. Plugging in the values, we get 1/-20 = 1/10 + 1/di, which simplifies to di = -6.7 cm. Since the image distance is negative, the image forms 6.7 cm behind the mirror, which is represented by choice D.

A Mirror Has a Focal Length −20 Cm