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  4. 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 distance 'd' between them. If a parallel beam of light falling on 'A' leaves 'B' as a parallel beam, then the distance 'd' in cm will be :

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Q. 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 distance $'d'$ between them. If a parallel beam of light falling on $'A'$ leaves $'B'$ as a parallel beam, then the distance $'d'$ in cm will be :

NEETNEET 2021Ray Optics and Optical Instruments

Solution:

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Point $F$ is the second focal length of $A$ and first focal length of $B$.
$\therefore d=(20-5) \,cm =15\, cm$