An image of an object obtained by a convex mirror is 4 times smaller than the object. If the focal length of the mirror is 20 cm, the object distance would be .

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(3) 60 cm. Given: convex mirror, f = +20 cm. A convex mirror always gives an erect virtual image, so '4 times smaller' means m = +1/4. Idea: m = f/(f-u) gets there in one step, without finding v first. 1/4 = (20)/(20-u), so 20-u = 80 and u = -60 cm. Check with the mirror formula: 1/v = 1/(20)+1/(60) = 4/(60), so v = 15 cm, behind the mirror and virtual, and m = -(15)/(-60) = +0.25. So the object stands 60 cm in front of the mirror.

An image of an object obtained by a convex mirror is 4 times smaller than the object. If the focal length of the mirror is 20 cm, the object distance would be ______.

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