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Question

A plano convex glass lens (μg=32) of radius of curvature R=10 cm is placed at a distance of b from a concave lens of focal length 20 cm. What should be the distance a of a point object O from the plano convex lens, such that the position of final image is always at same distance from concave lens?


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

The correct option is D 20 cm
Focal length of plano-convex lens is given by:


Using lens maker formula for plano convex lens,
1f=(μlμm1)(1R11R2)
1f=(321)(1101)
f=20 cm

If the object O is placed at a distance 20 cm from the plano-convex lens, the rays become parallel and the final image is formed at second focus or 20 cm from the concave lens which is independent of b.

Why this question:

This question checks the basic rules of image formation, for different positions of object.

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