Polarisation

In 1808, Etienne Louis Malus (1775-1812) observed the birefringence of a piece of calcite while looking at two images of the setting sun, one faint and the other relatively bright. He suggested and related the word Polarisation to this property of light, the orientation of the preferred direction of light to the poles of the earth. Malus assumed an arbitrary definition based on Newtonian theory to define the plane of Polarisation of the reflected light as the plane of incidence without regard to Electromagnetic theory.

What is Polarisation

Polarisation is a property of certain electromagnetic radiation in which the direction and magnitude of the vibrating electric field are related. Light waves are transversal: that is, the electrical vibration vector associated with each wave is perpendicular to the direction of propagation. A beam of unpolarized light consists of waves traveling in the same direction with their electrical vectors pointing at random orientations about the axis of propagation. Plane polarized light consists of waves in which the direction of vibration is the same for all waves. With circular Polarisation, the electric vector rotates around the direction of propagation as the wave propagates. Light can be polarized by reflection or by passing through filters such as certain crystals that transmit vibrations in one plane but not in others.

Various types of Polarisation

Mainly there are three types of polarisation exists, that are as follows

Linear Polarisation

When a light wave is linearly polarized, it refers to the electric field vibrating in a linear direction perpendicular to the axis of the wave. The magnetic field vibrates perpendicular to both the axis of advancement and the direction of the electric field. The direction of Polarisation is considered as the direction of vibration of the electric field. Polarisation can occur in any other direction perpendicular to the wave axis. Rotating the Polarisation by 180° does not lead to a rationally different state.

Circular Polarisation

Here, the electromagnetic field has constant amplitude at each point. But the direction rotates at a constant value in a plane perpendicular to the direction the path is in. A circularly polarized wave can rotate in two ways: either in the vector of the electric field which rotates according to the right-handed direction of propagation or according to the left-handed direction of propagation. The phenomenon of Polarisation results from the fact that light acts as a two-dimensional transverse wave.

Elliptical Polarisation

This type of Polarisation occurs when the peak of the electric field vector creates an ellipse normal to the direction of propagation and crosses any fixed plane. Two linearly polarized waves can be branched off from elliptically polarized waves.

Polarisation Method

There are four methods of polarisation, that we are going to discuss below

Polarisation by Refraction

Polarisation can also result from the refraction or bending of light as it travels from one medium to another. In most cases, Polarisation occurs perpendicular to the surface. If the refractive index of a material depends on the direction of incidence and Polarisation of the light, it is called birefringence. In birefringent materials, Polarisation splits an incident light beam into two beams within the material, which take slightly different paths.

Polarisation by Reflection

Unpolarized light can also be polarized by reflection from non-metallic surfaces. The degree of Polarisation depends on the angle at which the light hits the surface and the material the surface is made of. Metallic surfaces reflect light with a variety of vibrational directions, the reflected light is not polarized. However, non-metallic surfaces such as paved roads, snowfields, and water reflect light in such a way that there is a large concentration of vibrations in a plane parallel to the reflective surface.

Polarisation by Polaroid Filter

The most common Polarisation method involves the use of a Polaroid filter. Polaroid filters are made with a special material capable of blocking one of the two planes of vibration of an electromagnetic wave. In this sense, a Polaroid acts as a device that filters out half of the vibrations when transmitting light through the filter. When unpolarized light is transmitted through a Polaroid filter, it emerges with half the intensity and vibration on a single plane; it appears as polarized light.

Polarisation by Scattering

Polarisation also occurs when light is scattered as it travels through a medium. When light strikes the atoms of a material, it often vibrates the electrons in those atoms. The oscillating electrons generate their own electromagnetic wave, which radiates outwards in all directions. This newly created wave hits neighbouring atoms and forces their electrons to vibrate at the same original frequency. These oscillating electrons create another electromagnetic wave, which in turn radiates outwards in all directions. This absorption and re-emission of light waves causes light to be scattered. This scattered light is partially polarized. Scattering Polarisation is observed when light penetrates our atmosphere. The scattered light often creates a glow in the sky.

Conclusion 

In this article everything about Polarisation is discussed such as when the term Polarisation first came and what are the types of polarisation?, what are the methods of polarisation? You will get an idea about everything related to it. Moreover this article will help you in understanding the term Polarisation in detail. Hope this article is useful for you and you like this article.