Concepts of Physics

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

Laser Light is Polarized

Objective

To show that light from a laser torch is linearly polarized.

Introduction

The light from an ordinary source such as sun, incandescent bulb, candle etc is unpolarized. This means that the \(E\) field keeps changing its direction randomly in the plane perpendicular to the direction of propagation. But laser light from the laser torch is linearly polarized. To investigate the polarization properties one uses polarizing sheets. In these sheets the molecules are arranged in such a way that they absorb light if the vibration of \(E\) field is parallel to one particular direction in the sheet and allows light if the vibration of \(E\) field is perpendicular to this direction. If the linearly polarized light is made to fall perpendicular on a polarizer sheet and sheet is rotated in its plane, the intensity of transmitted light varies between almost zero to a maximum.

Apparatus

a laser torch, a polarizer

Procedure

Fix the laser torch in a stand. Let the beam fall on the wall or on the roof. Bring the polarizing sheet in front of the laser torch. The light passes through the sheet and then falls on the wall. Now rotate the sheet in its own plane slowly and look at the intensity of the spot. At certain orientation, you will find that the intensity falls to a very low value. As the sheet is rotated in either direction from such a position, the intensity increase and becomes maximum, when the sheet is kept at 90 degree from the minimum intensity position.

Discussion

This shows that laser light is polarized. Repeat the exercise after replacing laser torch by an ordinary torch. You will see that the intensity of the transmitted light does not change as the sheet is rotated. Accordingly, light from ordinary torch is unpolarized.