A candle is placed 20cm from the surface of a convex mirror and a plane mirror is also placed so that the virtual images in the two mirrors coincide. If the plane mirror is 12cm away from the object, what is the focal length of the convex mirror?
A
20cm
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B
15cm
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C
10cm
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D
5cm
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Solution
The correct option is D5cm For convex mirror, u+v=12×2=24cm (because for plane mirror, distance of object = distance of image). Also, here for the convex mirror u=20cm, therefore v=4cm. Hence, using 1v+1u=1f We find f=5cm.