A 6.0 cm tall object is placed 20 cm in front of a convex mirror with focal length −100 cm. Where is the image formed?
A) 80 cm behind the mirror
B) 25 cm behind the mirror
C) 17 cm behind the mirror
D) 17 cm in front of the mirror
E) 25 cm in front of the mirror

Question

Quizplus · Accepted Answer

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
Substituting the given values, we get,
1/-100 = 1/20 + 1/di
1/di = -1/-100 - 1/20
1/di = 0.01 - 0.05
1/di = -0.04
di = -25 cm (Negative sign indicates that the image is formed behind the mirror)
Using magnification formula,
m = -di/do
m = -(-25)/20
m = 1.25 (Positive sign indicates that the image is upright)
Since the image distance is negative, the image is formed behind the mirror at a distance of 25 cm from the mirror. Hence, the correct option is C.

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