Question

A convex mirror has a focal length of
-18 cm. How far behind the mirror does the image of a person 3.0 m away appear? The answer must be in cm.

The equation to use woulde be:
1/f = 1/do + 1/di

do = 300 cm
f = would it be -18 cm or 18 cm???

Answer 1

Use negatve numbers for the focal length of a convex mirror. They already tell you to use -18.

-1/18 = 1/300 + 1/di
1/di = -.058888
di = -16.98

Negative numbers for the di mean that the image is behind the mirror. This would be typical of a mirror used as a wide-angle side view mirror for a car, or for viewing aorund a corner.