2021 Ray Optics and Optical InstrumentsCombination of Lenses – Telescopic Arrangement Medium Question 24 of 89 in Ray Optics and Optical Instruments

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 15
B 50
C 30
D 25

The answer is A.

Solution


For a parallel beam to emerge parallel, the second focal point of A must coincide with the focal point of B (on the far side of B).
$d = f_1 - f_2 = 20 - 5 = 15$ cm

Concept separation for parallel emergent beam

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89 questions from this chapter have appeared across 25 exam years.