Polarisation of Light

What Is The Polarization of Light?

Sunlight and other natural, as well as artificial sources, give rise to light. Truly, light is a wave phenomenon. It can bend around objects. It can diffract and interfere. The light waves travel through the vacuum to reach us and because of Earth’s magnetic field, it becomes an Electromagnetic wave. These light waves are transverse. It exhibits the phenomenon of reflection, refraction, interference, diffraction, double refraction and polarisation. The electric and magnetic field vectors of the light wave travel in all directions. If the movement of these vectors is restricted to a single plane then the effect is called Polarization of light. Specialized materials are used to filter these beams, according to the direction of propagation. 

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Types of Polarization of Light

Depending upon the transverse and longitudinal wave movement, the polarization of light is classified into three types.

  1. Linear polarization

  2. Circular polarization

  3. Elliptical polarization

  1. Linear Polarization- Linearly polarized light wave means that the electric field vibrates in a certain linear direction perpendicular to the wave axis, and the magnetic field vibrates in a direction which is perpendicular to both, the advancement axis and direction of the electric field. The direction of polarization is considered to be the direction of the electric field vibration. The polarization can take place in any other direction perpendicular to the wave axis. Rotation of the polarization by 180° does not lead to a rationally different state.

  1. Circular Polarization- It is the type of polarization in which at every point, the electromagnetic field has a constant magnitude but its direction rotates with a constant value in a plane perpendicular to the direction of the wave. A circularly polarized wave can rotate in two ways, either the electric field vector rotates according to the direction of propagation in a right-hand direction or according to the direction of propagation in a left-hand sense. The phenomenon of polarization rises as a result of the fact that light acts as a 2 Dimensional transverse wave.

  1. Elliptical Polarization- It is the type of polarization where the tip of the electric field vector defines an ellipse in any fixed plane traversing, and normal to the direction of propagation. An elliptically polarized wave may be bifurcated into two linearly polarized waves with their polarization planes perpendicular to each other. As the electric field can rotate clockwise or anti-clockwise while propagating, elliptically polarized waves show chirality.

Circular polarization and linear polarization can be considered as the special cases of elliptical polarization.

Methods of Polarization of Light

There are various methods of Polarization of the light, but the following four methods are more popular.

1) Polarization by Scattering,

2) Polarization by Reflection,

3) Polarization by Refraction &

4) Polarization by Transmission.

  1. Polarization by Scattering- When light travels through a medium, atoms of the medium (also the dust present in the medium) vibrate and produce electromagnetic waves. These waves are radiated outwards and thus the light is scattered. In this entire process, absorption and remission of light waves occur throughout the material. The scattered light is also known as partially polarized. Transmission of these partially polarised lights causes glare.

  1. Polarization by Reflection- When unpolarized light is made to fall on a non-metallic surface, at a particular angle, the surface reflects the polarized light. In this process, the angle of incidence and the non-metallic surface plays an important role to regulate the magnitude of polarization.

  1. Polarization by Refraction- Refraction is when a light wave travels from one medium to another, it changes its direction and speed. This refracted beam attains some degree of polarization. In the majority of the cases, polarization by refraction occurs in the plane which is perpendicular to the surface.

  1. Polarization by Transmission- In this method, involves the use of filter materials that have special chemical composition. They are known as Polaroid filters. These polaroid filters can block one of the two planes of electromagnetic waves. When the unpolarized light is transmitted through these polaroid filters, it filters out the one-half of the vibrations of the light in a single plane. This polarized light has one half of the intensity. 

Applications of Polarization of Light

  1. In the production of glare-reducing sunglasses.

  2. In many industries, polaroid filters are used for stress analysis tests on transparent plastics.

  3. In the entertainment industry, to telecast or to show a 3D film, the phenomenon of polarization of light is used.

  4. To differentiate between a longitudinal and a transverse wave.

  5. To cut the refractions, Fishermen, Skiers, motorists, sportsmen need special sunglasses. In the production of these special sunglasses polarization of light is used.

  6. Many photographers use polarizers to capture the perfect scene.

  7. Polarization microscopes are used in geological studies to identify the minerals.

  8. In infrared spectroscopy.

  9. To check the chirality of organic compounds

  10. To study the physics of the early universe, the effect of polarization is used.

  11. To know the source of radiation and scattering.

  12. To characterize the stress-strain distribution in the prototypes, polarization is used.

  13. To identify thermally tempered lenses.

  14. In ophthalmic instruments, to eliminate strong reflection from a patient's cornea, the phenomenon polarization of light is used.

  15. As light passes through the atmosphere polarization of light is observed

  16. The scattered light is known to give rise to various colours in the sky 

FAQ (Frequently Asked Questions)

1. What does ‘the light from the sun is unpolarised’ means and how does it get polarized?

Ans. It means that the light emitted by the sun travels in all the given directions i.e in different planes. And while being transmitted through a distance it gets semi polarized and only gets polarized when its angle of reflection is equal to the angle of polarization. Because the light of the sun takes all directions, it is said to be unpolarized light. When unpolarized light falls on the transparent surface at an angle of incidence equal to the polarization angle or also called Brewster's angle, it is called as plane-polarized. When the unpolarized light is passed through a polarizing sheet, it becomes polarized.

2. Mention different types of polarization?

Ans. Different types of polarization are:

  • Electronic polarization: The displacement occurring in dielectric elements and minerals between a positive charge and negative charge results into Electronic polarization.

  • Linear polarization: It means that light is confined to one plane only.

  • Space charge polarization: It means the accumulation of the charge at electrodes and their interface.

  • Ionic polarization: It occurs in dielectric metals which have atomic bonds between them.