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 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 (a). 25 (b). 15 (c). 30 (d). 50

5 cm is the focal length of a concave lens, i.e., if the lens is placed at 5 cm from F2 towards the left, the beam will become parallel.

In the figure, the separation between the lenses is clear

d = 20 - 5 = 15 cm

Therefore, the distance d in cm will be 15.

Summary:

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 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 (a). 25 (b). 15 (c). 30 (d). 50

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 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 15. 

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