Editorial Feature

Methods Employed to Polarize Light


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Unpolarized light can be transformed into polarized light. Light is a transverse, electromagnetic wave and if its electric field is confined to a single plane along the direction of propagation, then this is referred to as linearly polarized light. The transformation of unpolarized light into polarized light is called polarization.

Several methods have been used to polarize light. The four most common methods are:

  • Polarization by using Polaroid Filter
  • Polarization by Reflection
  • Polarization by Refraction
  • Polarization by Scattering

Polarization by Using Polaroid or Dichroic Filters

This is the most common method for polarizing light. Polaroid filters consist of special, dichroic materials that absorb specific polarizations of light and transmit all the rest.

Polaroid filters are often made from iodine-impregnated polyvinyl alcohol. The sheets of the material are stretched while they are softened to allow alignment of the molecules along an axis. As the iodine atoms can only move up and down the chains, not perpendicular to them, this allows for strong absorption of light aligned parallel to the chains. However, light polarized in the perpendicular direction can still pass through. 

Polarization by Reflection

Non-metallic surfaces can also be used to polarize unpolarized light. This method is commonly known as polarization by reflection. The angle at which the light approaches the surface and the material that the surface is made of governs the extent of the polarization process and how selectively particular polarization detections are reflected. This is not the case for a metallic surface which will reflect light with nearly the same polarization properties as the incident light. 

Polarization by Refraction

Polarization can also be carried out by the refraction of light. When a light beam passes from one material into another material and undergoes a change in speed and therefore direction, this is known as refraction. The refracted beam acquires some degree of polarization. In most cases, polarization occurs in a plane that is perpendicular to the surface.

The polarization of refracted light is sometimes demonstrated using a type of crystal which serves as a double-refracting crystal. Such materials are also known as birefringent.

Polarization by Scattering

Polarization can also occur when light is scattered, such as when it travels through a medium. This is because the scattering of the incident wave by the atoms in the material causes a vibration of the electrons, which in turn will generate electromagnetic waves, radiated outwards in all directions. Absorption and reemission of the radiation would occur throughout the material and the scattered light known to be partially polarized.

This article was updated on the 28th January, 2020

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