A plano convex lens of refractive index 1.5 and radius of curvature 30 cm is silvered at the curved surface. Now this lens has been used to from the image of an object. At what distance from this lens an object be placed in order to have a real image of the size of the object :

20 cm

30 cm

60 cm

80 cm

A

30 cm

B

80 cm

C

20 cm

D

60 cm

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Solution

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If the curved surface of a plano-convex lens is silvered, the focal length of the lens is given by

1F=2μR

⟹F=R2μ=302×1.5=10cm

The real image will be equal to the size of the object if the object distance u=2F=2×10=20cm

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