Q132 / 200 · NEET 2021 · Physics

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 :

A
$50$
Correct answer
B
$15$
C
$25$
D
$30$

Solution

\frac{1}{F} = \frac{1}{f_{1}} + \frac{1}{f_{2}} - \frac{d}{f_{1} f_{2}}

∴ \frac{1}{\infty} = \frac{1}{20} + \frac{1}{- 5} - \frac{d}{20 (- 5)}

\Rightarrow 0 = \frac{1}{20} - \frac{1}{5} + \frac{d}{100}

∴ \frac{d}{100} = \frac{1}{5} - \frac{1}{20} = \frac{20 - 5}{100}

∴ d = 15 c m

From figure it is clear that separation between lenses, $d = 20 - 5 = 15 cm$

Hence, the correct answer is option: 'B'.

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