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Question

A convex lens of focal length 20 cm and a concave lens of focal length f are mounted coaxially 5 cm apart. Parallel beam of light incident on the convex lens emerges from the concave lens as a parallel beam. Then, f in cm is

A
35
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B
25
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C
20
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D
15
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Solution

The correct option is D 15
1ffinal=1f1+1f2xf1f2

as the rays from infinity travel undeviated ffinal is

0=1f1+1f2xf1f2

0=120+1f520f

f=15

option D is correct

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