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Understanding the Polarisation of Light

Most of us are familiar with Polaroid glasses. These glasses have an uncommon ability to cut the glare of light reflected from glass or water. Polaroids possess this ability because of a wave characteristic of light called polarisation. In this article, we will help you get acquainted with the phenomenon of polarisation and some of its uses.

What is the Polarisation of Light?

Sunlight and other forms of light sources produce light waves whose electric field vectors vibrate in all planes that are perpendicular with respect to the direction of propagation. If these electric field vectors are restricted to only a single plane by filtration of the beam with specialized materials, then the light is referred to as a plane or linearly polarized.

The light emitted by the sun, by a lamp or a candle is unpolarized. It is possible to transform unpolarized light into polarized light. This process of transformation of unpolarized light into polarized light is known as polarisation. Following are the three methods by which polarisation is achieved:

  • Polarisation by Reflection – If light strikes an interface such that there is a 90o angle between the reflected and refracted rays, the reflected light will be linearly polarized.
  • Polarisation by Refraction – Polarisation can occur by the refraction of light. The refracted beam acquires some degree of polarisation.
  • Polarisation by scattering – Polarisation also occurs when light is scattered through a medium.

Types of Polarisation

There are three types of polarisation as follows:

  • Linear polarisation
  • Circular polarisation
  • Elliptical polarisation

In linear polarisation, the electric field is limited to a single plane along the direction of propagation. Circular polarisation is a type of polarisation where either the electric or the magnetic vector executes a circle perpendicular to the path of distribution with a frequency equal to that of the wave. Elliptical polarisation is the polarisation of electromagnetic radiation such that the tip of the electric field vector describes an ellipse in any fixed plane intersecting, and normal to, the direction of propagation.

Applications of Polarisation

Polarisation has a wealth of applications besides their use in glare-reducing sunglasses. Below we have listed a few of these applications:

  • Polarisation is used in the entertainment industry to produce and show 3-D movies.
  • Polarisation is used in sunglasses to reduce glare.
  • They are used in industries to perform stress analysis tests.
  • The chirality of organic compounds is tested using polarisation techniques.
  • Polarisation is used to differentiate transverse and longitudinal waves.

What is a Polaroid Device?

A Polaroid device is a device that is used to produce plane-polarized light. It comprises ultramicroscopic crystals of quinine iodo sulphate embedded in nitrocellulose films in a way that their optic axes are parallel to each other.

Polarisation is a phenomenon peculiar to transverse waves. Longitudinal waves, such as sound cannot be polarized. This is because, in a longitudinal waves, the direction of vibration and direction of propagation is the same. This is one of the reasons why scientists believe that light exhibits wavelike behaviour.