Polarisation of light

Polarisation is a phenomenon generated by the wave nature of electromagnetic radiation, according to physics. Sunlight is an example of an electromagnetic wave that travels through the vacuum to reach the Earth. When an electric field interacts with a magnetic field the waves that are produced are known as electromagnetic waves. We will learn about two types of waves in this article: transverse waves and longitudinal waves. 

Type of Waves:

There are two types of waves that are differentiated on the basis of the motion of the them:

• Transverse Wave:

The waves in which the movement of the particles is perpendicular to the wave’s motion direction.

Common example of a transverse wave is when you throw a stone, it creates ripples in the water.

• Longitudinal Wave:

The waves which occur when the medium’s particles move in the same direction as the waves.

The most common example of longitudinal waves is sound waves.

Light:

The interaction of electric and magnetic fields travelling across space is known as light. The electric and magnetic fields are perpendicular to each other. The magnetic field goes in one direction and the electric field in the other, but they are always perpendicular. So, we have an electric field in one plane, a magnetic field perpendicular to it, and a travel direction that is perpendicular to both. Electric and magnetic vibrations can happen in a variety of planes.

A polarised wave is the other type of wave. Light waves that vibrate in a single plane are known as polarised waves. Plane polarised light is made up of waves in which the waves vibrate only in a single plane.  Polarisation is the process of conversion of unpolarised light to polarised light. The equipment used to polarise light is known as a polarizer.

Types of Polarisation:

There are three types of polarisation that depends on longitudinal or transverse wave motion these are:

• Linear Polarisation
• Circular Polarisation
• Elliptical polarisation

Linear Polarisation:

The polarised light in which the electric field of light is restricted to a single plane along the direction of propagation.

Circular Polarisation:

In Circular polarisation the electric field of light has two linear components that are perpendicular to each other and have identical amplitudes, but the phase difference is π/2. The electric field that occurs will propagate in a circular motion.

Elliptical Polarisation:

The electric field of light that propagates in an elliptical path. The two linear components do not have the same amplitude and phase difference.

Methods of Polarising light:

The methods that are used for polarising light waves are:

1. Polarisation by Transmission
2. Polarisation by Reflection
3. Polarisation by Scattering
4. Polarisation by Refraction

Polarisation by transmission:

In this method, a Polaroid is a used that filters out half of the vibrations transmitted through the filter as light passes through it. When unpolarized light passes through a Polaroid filter, it is reduced to half its original intensity and vibrates in a single plane, resulting in polarised light.

Polarisation by reflection:

When a light beam strikes a material’s surface, some of the light is reflected and some is refracted (it travels through the material). Brewster’s angle is the angle of incident light required for the reflected and refracted light to be at a perfect right angle.

The incident light will induce linear polarisation of the reflected light when the angle of incidence is equal to Brewster’s angle (depending on the compositions of the mediums on each side of the surface) and the incident light is unpolarized. If the incident light has a polarisation that is unique to the material, it will only be refracted, leaving no reflected light. 

Polarisation by Scattering:

Light is linearly polarised perpendicular to the plane of incidence due to scattering of incident light by air molecules. Air molecules have their own little oscillation in one direction, called a dipole moment, and they emit energy perpendicular to that oscillation’s line. As a result, if a molecule’s dipole moment oscillates back and forth on the y-axis, incident unpolarized light scatters in the x-direction whereas polarised light scatters in the y-direction (parallel to the dipole).

Rayleigh scattering occurs when the wavelength of the light is similar to the size of the molecules. Rayleigh scattering is responsible for the sky’s hue, whether it’s a deep blue on a sunny day or a deep red on a stormy night.

Polarisation by Refraction:

Refraction, or the bending of light as it moves from one medium to another, can also cause polarisation. Polarisation happens most often perpendicular to the surface.

Birefringent means that the refractive index of a material changes depending on the incident direction and polarisation of the light. An incident ray of light is split into two rays inside birefringent materials by polarisation, which adopt slightly different trajectories.

Applications of Polarisation:

• Polarisation is used to reduce glare in sunglasses.
• Polaroid filters are used to conduct stress analysis testing in the plastic industry.
• It is used in differentiating between transverse and longitudinal waves via polarisation.
• In the medical field it is used in infrared spectroscopy.
• Earthquake research in seismology is done using polarisation.
• The determination of chirality of organic molecules is done using polarisation techniques.

Conclusion:

The orientation of the plane of the disturbance in a transverse wave is referred to as polarisation. All electromagnetic waves, including light, can be polarised. Longitudinal waves, such as sound, cannot be polarised. If the polarisation of a transverse wave varies rapidly and arbitrarily, it is said to have no polarisation or to be unpolarized.