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Question

# A concave mirror has a focal length of 20 cm. The distance between the two positions of the object for which the image size is double of the object size is

A
20cm
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B
40cm
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C
30cm
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D
60cm
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Solution

## The correct option is A 20cmThe focal length of the mirror is 20cm. In the case of real image, The object distance is u1, then the image distance will be −2u1. The mirror formula is given as, 1v+1u=1f 1−2u1−1u1=−120 u1=30cm In the case of virtual image, The object distance is −u2, then the image distance will be 2u2. The mirror formula is given as, 1v+1u=1f 12u1+1−u1=−120 u2=10cm The distance between two positions of object is given as, u=30−10 =20cm Thus the distance between the two positions of object is 20cm.

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