Question

A concave lens has focal length 15cm. At what distance should the object from the lens be placed so that it forms an image at 20cm from the lens?​

Answer 1

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For a concave mirror, image size is same as the object size when it is placed at the center of curvature. Since radius of the mirror is twice the focal length, center of curvature lies at a distance of 30 cm from the pole. Hence, an object kept at 30 cm will produce an image of the same size as that of the object.

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Answer 2

For a concave mirror, image size is same as the object size when it is placed at the center of curvature. Since radius of the mirror is twice the focal length, center of curvature lies at a distance of 30 cm from the pole. Hence, an object kept at 30 cm will produce an image of the same size as that of the object.