A convex lens A with a focal length of 20 cm and a concave lens B with a focal length of 5 cm is kept along the same axis with the distance d between them. If a parallel beam of light falling on A leaves B as a parallel beam, then distance d in cm will be

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Solution

The correct option is B. 15.
In the absence of a concave lens, the parallel beam will be focussed at the focus F2 of the convex lens i.e. at a distance of 20 cm. from lens A. The focal length of the concave lens is 5 cm. i.e. if this lens is placed at 5 cm from F2 towards the left, then the beam will become parallel.
From the figure, it is clear that the separation between lenses
d = 20-5 = 15 cm.

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A convex lens A of focal length 20 cm and a concave lens B of focal length 5 cm are kept along the same axis with a distace d between them. If a parallel beam of light falling on A leaves B as a parallel beam, then d is equal to
(IIT-JEE 1985)

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