A concave mirror gives an image three times as large as the object placed at a distance of 20 cm from it. For the image to be real, the focal length should be

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–10 cm

–15 cm

–20 cm

–30 cm

answer is B.

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Detailed Solution

Object distance, u = –20 cm , As image is real, thus , m = –3∵m=−vu⇒−3=−v−20⇒v=−60cm1f=1u+1v so f=-15 cm

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