Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Divergence by Convex Lens

Objective

To get more familiar with geometrical optic.

Introduction

Convex lens is known for converging a beam of light.

Apparatus

Convex lens, laser torch

Procedure

Take a laser torch and direct it towards a distant wall or a roof. Laser goes as a parallel beam and make a spot of small size. Now, ask the audience what will happen to the spot if we bring a convex lens in between the laser torch and the spot. The general reply is that the beam will converge and spot size will be reduced.

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Now, take a convex lens and put it in front of the laser torch. The spot size will increase and the audience will feel that beam is diverging.

Discussion

As is clear from the figure, if the wall is too far away from the lens, the spot size will become larger. The lens converges the parallel beam to its focus and beyond the focus the rays continues and hence spot becomes larger.