A convex lens A of focal length 20 cm an a concave lens B of focal length 56 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 distances d in cm will be

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answer is B.

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Detailed Solution

$\begin{array}{l} P = \frac{1}{f_{1}} + \frac{1}{f_{2}} - \frac{d}{f_{1} f_{2}} = 0 \\ ∴ \frac{1}{f_{1}} + \frac{1}{f_{2}} = \frac{d}{f_{1} f_{2}} \\ \frac{1}{20} - \frac{1}{56} = \frac{d}{20 (- 56)} \\ \frac{56 - 20}{20 \times 56} = \frac{d}{- 20 \times 56} \\ d = - 36 c m \end{array}$

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