First slide

Ray optics

Question

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

Easy

A

25
B

15
C

30
D

50

Solution

$\begin{array}{l} P = \frac{1}{f_{1}} + \frac{1}{f_{2}} = \frac{d}{f_{1} f_{2}} \\ 0 = \frac{1}{20} - \frac{1}{5} - \frac{d}{20 (- 5)} \\ \frac{d}{100} = \frac{1}{5} - \frac{1}{20} = \frac{4 - 1}{20} = \frac{3}{20} \\ or d = 15 cm \end{array}$

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