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>> $A convex lens A of focal length 20cm and$

Question

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

A

$15$

B

$25$

C

$30$

D

$50$

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Solution

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Correct option is A)

$d = f_{1} - f_{2}$
By using the formula
$\frac{1}{f _{1}} + \frac{1}{f _{1}} - \frac{d}{f _{1} f _{2}} = \frac{1}{F}$
For the emergent beam to parallel
$P = 0, F = α$
$\Rightarrow \frac{1}{2 0} - \frac{1}{5} + \frac{d}{1 0 0} = 0$
$\Rightarrow$ $d = 15$

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