Polarisation

In this article, we are going to study about a phenomenon related to light waves – polarisation.

Table of Contents

• What is Polarisation?
• What is a Simple Polaroid?
• Applications of Polaroids

What is Polarisation?

Light waves are transverse in nature. It means that the electric field associated with a propagating light wave is always at right angles to the direction of propagation of the wave.

In any ordinary light, the electric vectors associated with this electric field are spread in all possible directions in a plane that is perpendicular to the direction of propagation of light. The phenomenon of polarization is related to restricting these electric vectors in a single direction.

This is often achieved using polaroid.

Note

Polarization is a phenomenon that is related only with transverse waves, such as light waves. It does not occur with longitudinal waves, such as sound waves.

What is a Simple Polaroid?

A simple polaroid has long chain of molecules which are all aligned in a particular direction. If an ordinary light is incident on such a polaroid, then:

• The electric vectors along the direction of the aligned molecules get absorbed.
• The electric vectors perpendicular to the direction of the aligned molecules pass through.

So, the light coming out of such a simple polaroid will be linearly polarised, with the electric vector oscillating along a direction perpendicular to the aligned molecules.

The linear direction in which the light passing from a simple polaroid oscillates is called the Pass-Axis of that polaroid.

Interaction of Two Polaroids

When an ordinary light (from an ordinary source like a sodium lamp) passes through a simple polaroid, its intensity reduces by half. Say, if the original intensity of light was x, it will be x/2 now.

Now, if another simple polaroid is kept in the way of the linearly polarised light coming out of the first simple polaroid, we can easily control the intensity of the light emerging from the second polaroid by adjusting the angle between the pass-axes of two polaroids.

So, the intensity of the light coming out of the second polaroid may vary anywhere between x/2 and zero.

Applications of Polaroids

Now, let’s have a look at some of the use cases of polaroid.

• As already mentioned, Polaroids are used to produce and analyse a plane-polarised light.
• Polaroids are used to cut off excessive glare and to control the intensity of light entering inside, e.g. in head-lights and wind-screens of cars, sun glasses, trains, aeroplanes, etc. For example, a polaroid is fixed outside the window and the other inside it. A passenger may rotate the inner polaroid and thus control the intensity of light entering inside.
• Polaroid glasses are used in viewing three-dimensional pictures.