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Question

# The convex surface of a thin concavo-convex lens of glass of refractive index 1.5 has a radius of curvature 20cm. The concave surface has a radius of curvature 60 cm. the convex side is silvered and placed on a horizontal surface. Where should a pin be placed on the optic axis such that its image is formed at the same place?

A
15 cm
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B
30 cm
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C
40 cm
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D
45 cm
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Solution

## The correct option is A 15 cm For lens, 1fy=(n−1)[1R1−1R2]=(1.5−1)[1120−160]=160 ⇒fg=60cm and fm=R12=202=10 cm1F=160+110+160=860⇒F=608=7.5 cm For the image to be formed at the place of the object, X=R=2F=7.5×2=15 cm

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