Topic: Magnification, power of lens, combination of thin lenses· Optics

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 w…

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 :

1. 25

2. 15

3. 50

4. 30

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Answer: 2. 15

Key idea: a parallel beam stays parallel only if the lens separation makes the focal points coincide.

The convex lens A focuses the beam at 20 cm behind it. For the concave lens B to give a parallel beam, this point must be its focus (virtual object at its focal point on the far side). So \(d = f_A - |f_B| = 20-5 = 15\) cm. Option 2.

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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 w…

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