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 from of the mirror
D) 6.7 behind the mirror
E) no image will form

Question

Quizplus · Accepted Answer

Using the mirror equation 1/f = 1/di + 1/do, where f is the focal length, di is the image distance, and do is the object distance, we can solve for di: 
1/-20 = 1/di + 1/10 
-1/di = -3/20 
di = -20/3 ≈ -6.7 cm 
The negative sign indicates that the image is formed behind the mirror, so the answer is D) 6.7 cm behind the mirror.

A Mirror Has a Focal Length -20 Cm