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 a distance ′d′ between them as shown in a figure. If a parallel beam falling on A emerges parallel from B, then ′d′ is equal to

A
15
B
20
C
22.5
D
25
Solution
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#### 1V=1U=1fAs the incident beam is parallel; (Let the there be no second lens), the image will form at V such that1V−1−∞=120V=+20cm(i) if d is the distance between two lenses, then distance between the image and the concave lens is (20−d)(ii) Image will act as object for 2nd refraction and image will be at infinity.1∞−1(20−d)=1−5(20−d)=−5d=15cm

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