Distance between Lenses for Parallel Beam Output – Rankers Physics

Refraction by Spherical Surfaces: Practice Problem & Solution

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
30
25
15
50

Short Solution (Best for NEET):

To solve this problem, we apply the core principles of Refraction by Spherical Surfaces. Understanding the underlying formula is key to arriving at the correct answer below:

For an incoming parallel beam to emerge parallel from the combination, the focus of the first lens must coincide with the virtual focus of the second lens. Thus, \(d = f_1 - |f_2| = 20 - 5 = 15\) cm.

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