Polarisation is defined as a phenomenon in which the vibrations of a transverse wave are wholly or partially restricted in one particular direction. Do you know which device is used to edit images like darkening the skies, manage reflections or suppressing the glare from the surface of lakes or the sea in a camera?

The device used in this photography application is a “polarising filter”. A polarising filter is a device that allows light to pass only if its vibration is in a certain direction. We can create light that vibrates just up and down or side to side by making it go through a polarising filter. Polarisation proves that light is a transverse wave. This article will explain about types of waves, polarisation phenomena, their types, and applications.

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Types of Waves

Waves can be of two types depending on the direction of wave propagation and the direction of vibration of particles of a medium. They are:
1. Transverse waves
2. Longitudinal waves

Transverse waves are waves in which the movement of the particles in the wave is perpendicular to the direction of motion of the wave.
Example: Ripples in water, light waves (Electromagnetic waves).

Longitudinal waves are one in which the particles of the medium travel in the direction of motion of the waves.
Example: The motion of sound waves through the air.

Types of Light Waves (Electromagnetic waves)

Light is an electromagnetic wave in which the electric and magnetic vibrations are perpendicular to each other. We have one plane occupied by an electric field, the magnetic field perpendicular to it, and the direction of travel perpendicular to both.

Light can be divided into two types depending upon the direction of vibrations of the electric field. They are:
1. Unpolarised Light
2. Polarised Light

1. Unpolarised Light

The Electromagnetic waves (light) having electric field oscillations in all directions in the plane perpendicular to the direction of wave propagation is called unpolarised light.

Natural light (sunlight) and artificial light sources (L.E.D.’s, incandescent lights, fluorescent lights, etc.) are unpolarised. Sunlight travels in any radial direction from the source of the light. Imagine a beam of light. Light waves oscillate \({360^{\rm{o}}}\) from every point along that beam. Since the vibrations of the electric field can be in any direction, it is called unpolarised light.

2. Polarised Light

Polarised light is one in which the light waves travel in only one specific direction. The light waves having oscillations only in one plane is called polarised or plane polarised light. Light can be polarised in nature by refraction, reflection, absorption, scattering and birefringence (double refraction). For example, when light passes through a Polariser crystal, it passes through the polarizer and gets polarised in a specific direction which we experience as glare.

As the sun moves across the sky, the angle of the sun rays striking a window will change. At some point, the light will reflect from the window and gets polarised at an angle perceived as glare.

Note:
1. The plane of oscillation in the polarised light is the plane of oscillation.
2. The plane perpendicular to the plane of oscillation is the plane of polarisation.
3. Light can be polarised by transmission through certain crystals such as tourmaline, calcite, quartz, or polarisers.

Types of Polarisation

Depending on the wave motion, there are three types of polarisation. They are:
1. Linear polarisation
2. Circular polarisation
3. Elliptical polarisation

1. Linear Polarisation: Linearly polarised light is one in which the electric field vector vibrates in a certain linear direction perpendicular to the axis of the wave, and the magnetic field vibrates in a direction that is perpendicular to both the wave propagation direction and direction of the electric field. The direction of polarisation is usually considered as the direction of the electric field vector vibration. The polarisation can take place in a direction perpendicular to wave propagation. Rotation of the polarisation axis by \({180^{\rm{o}}}\) does not lead to a different state of vibration.

2. Circular Polarisation: In this type of polarisation, the electric and magnetic field has a constant magnitude at every point. But their direction rotates with a constant value in a plane perpendicular to the direction of the wave propagation. A circularly polarised wave can turn in different ways. The electric field vector rotates in clockwise sense or anti-clockwise sense. Polarisation rises as a result of the fact that light acts as a two-dimensional transverse wave.

3. Elliptical Polarisation: In this type of polarisation, the electric field vector moves in the form of an ellipse in a fixed plane. As the electric field vector can rotate clockwise or anti-clockwise while propagating, elliptically polarised waves show chirality.

Methods of Polarisation of Light

Following are the various methods of Polarisation of the Light:
1. Polarisation by Scattering
2. Polarisation by Reflection
3. Polarisation by Refraction
4. Polarisation by Transmission

1. Polarisation by Scattering: When light travels through a particular medium, molecules of the medium vibrate and produce electromagnetic waves. These waves are radiated outwards and the light is scattered. The scattered light is known as partially plane polarised light. Partially polarised lights cause glare on transmission.

2. Polarisation by Reflection: When unpolarised light is made to fall on a non-metallic surface, at a particular angle called polarising angle, the surface reflects the polarised light. In the process, the angle of incidence and the nature of the surface play an important role to regulate the magnitude of polarisation.

3. Polarisation by Refraction: Refraction occurs when a light wave travels from one medium to another of different refractive indices. Light changes its direction and speed. The refracted beam attains some degree of polarisation (partially plane polarised). Polarisation by refraction occurs in the plane perpendicular to the surface in most cases.

4. Polarisation by Transmission: This method involves using filter materials (Polariser) that have special chemical compositions. They are known as Polaroid filters. These Polaroid filters allow only one of the two planes of electromagnetic waves. The polarised light has one-half of the intensity because Polaroids filter out one-half of the vibrations of the light in a particular plane.

Polaroids

Etienne Malus discovered polarisation in \(1808.\) Malus’s law gives the intensity of light after an ideal polariser has polarised it. The origin of polaroids dates back to the \(1850\)s. Polaroids produce the plane polarised light. They are based on selective absorption and are more effective than the tourmaline crystal or quartz. Polaroids contain a thin film of ultramicroscopic crystals of quinine iodosulphate with their optic axis parallel.

1. Polaroids allow the light oscillations parallel to the transmission axis only.
2. The Polaroid crystal on which unpolarised light is incident is called Polariser. Polaroid crystal on which polarised light is incident is called analyser.

Case 1

Case 2

Note: When unpolarised light falls on Polaroid, the intensity of the transmitted polarised light is half the intensity of incident unpolarised light.

Malus Law

This law given by Étienne-Louis Malus states that the intensity of the polarised light transmitted through the analyser is directly proportional to the square of the cosine of the angle between the transmission plane of the analyser and the plane of the Polariser.

(i) \(I = {I_0}{\cos ^2}\theta \) and \({A^2} = A_0^2{\cos ^2}\theta \Rightarrow A = {A_0}\cos \theta \)
If \({A^2} = A_0^2{\cos ^2}\theta \Rightarrow A = {A_0}\cos \theta \) If \(\theta = {45^{\rm{o}}},\,I = \frac{{{I_0}}}{2},\,A = \frac{{{A_0}}}{{\sqrt 2 }},\) If \(\theta = {90^{\rm{o}}},\,I = 0,\,A = 0\)
(ii) If \({I_i} = \) Intensity of un-polarised Light, then \({I_0} = \frac{{{I_i}}}{2},\) i.e. if an unpolarised light is converted into plane polarised light (by passing it through a polaroid or a Nicol-prism), its intensity becomes half. and \(I = \frac{{{I_i}}}{2}{\cos ^2}\theta \)

Note: Percentage of polarisation \( = \frac{{\left( {{I_{\max }} – {I_{\min }}} \right)}}{{\left( {{I_{\max }} + {I_{\min }}} \right)}} \times 100\)

Brewster’s Law

Sir David Brewster discovered Brewster’s law of polarisation.

According to Brewster, when unpolarised light is reflected from a transparent medium of refractive index \( = \mu ,\) the reflected light is completely plane polarised when light is incident at a certain angle of incidence called the polarisation angle \({\theta _B}.\)

Note:
1. When \(i < {\theta _B}\) or \(i > {\theta _B}.\) Both reflected and refracted rays becomes partially polarised.
2. For glass \({\theta _B} \approx {57^{\rm{o}}},\) for water \({\theta _B} \approx {53^{\rm{o}}}.\)

Derivation of Brewster’s Law

AO – incident ray in the air (unpolarised),
OB – reflected ray (polarised)
OC – refracted ray (partially plane polarised)
\(i = {\theta _B} = \)angle of incidence, \(r = \)angle of refraction

The Polarising angle or Brewster’s angle is the angle of incidence at which the reflected ray is normal(perpendicular) to the refracted ray and the reflected beam is completely plane polarised.

Brewster’s law states that “When a ray reflected from a surface of a transparent object is completely plane polarised, the tangent of the angle of incidence or angle of polarisation is equal to the refractive index \(\left( n \right)\) of the material of the medium, i.e. \(n = \tan {\left( {{\theta _B}} \right)}.\)

Referring to the above diagram, when \(i = {\theta _B}\,\,\angle BOC = {90^{\rm{o}}}\) i.e., at the polarising angle of incidence, the reflected ray is at right angles to the refracted ray.
Therefore \({\theta _B} + r = {90^{\rm{o}}}\) or \(r = {90^{\rm{o}}} – {\theta _B}\)
From Snell’s law, the refractive index of the medium,
\(n = \frac{{\sin \,i}}{{\sin \,r}} = \frac{{\sin \,{\theta _B}}}{{\sin \left( {{{90}^{\rm{o}}} – {\theta _B}} \right)}} = \frac{{\sin \,{\theta _B}}}{{\cos \,{\theta _B}}} = \tan \,{\theta _B}\)
Therefore \(n = \tan \,{\theta _B}\)
Note:
1. In general, for a pair of media \(\tan \,{\theta _B} = \frac{{{n_2}}}{{{n_1}}}\)
2. Brewster’s angle of medium is least for red colour and maximum for the violet colour of light.

Applications of Polarisation

1. By calculating the polarising angle \({\theta _B}\) and using Brewster’s law, i.e. \(n = \tan \,{\theta _B},\) the refractive index of dark transparent substance can be determined.
2. It is used to reduce glare.
3. In calculators and watches, numbers and letters are formed by liquid crystals through the polarisation of light called LCD(liquid crystal display).
4. In a CD player, a Polarised laser beam serves as a needle for producing sound from a compact disc in an encoded format (digital).
5. It is also used in recording and reproducing three-dimensional \(\left( {3 – D} \right)\) pictures.
6. Polarisation of scattered sunlight is used for navigation in solar compasses in Polar Regions on earth.
7. Polarised light is used in the optical stress analyser method known as ‘photoelasticity’.
8. Polarisation is also used to study asymmetries in molecules and crystals through the phenomenon of ‘optical activity’.

Summary

1. Polarisation is defined as a phenomenon caused due to the wave nature of electromagnetic radiation. The vibrations of a transverse wave, especially Light, are wholly or partially restricted in one particular direction.
2. Malus law explains that the intensity of the polarised light transmitted through the analyser is directly proportional to the square of the cosine of the angle between the transmission plane of the analyser and the plane of the Polariser.
3. Brewster’s law states that “When a ray reflected from a surface of a transparent object is totally plane polarised, the tangent of the angle of incidence or angle of polarisation is equal to the refractive index of the transparent medium, i.e. \(n = \tan \left( {{\theta _B}} \right).\)
4. The polarisation phenomenon proves that light is a transverse wave.

FAQs on Polarisation

Q.1. What is the polarisation of light?
Ans: Polarisation is defined as a phenomenon caused due by the wave nature of electromagnetic radiation due to which the vibrations of a transverse wave, especially light, is wholly or partially restricted in one particular direction.

Q.2. What are the different types of light?
Ans: Light can be divided into two types depending upon the direction of vibrations of the electric field. They are:
1. Unpolarised Light
2. Polarised Light.

Q.3. What are the different types of polarisation of light?
Ans: There are three types of polarisation. They are:
1. Linear polarisation
2. Circular polarisation
3. Elliptical polarisation.

Q.4. What are the different methods of polarisation of light?
Ans: Following are the various methods of polarization of the light:
1. Polarisation by Scattering
2. Polarisation by Reflection and Refraction
3. Polarisation by Transmission and double refraction.

Q.5. Can sound waves undergo polarisation?
Ans: No, sound waves don’t undergo polarisation because they are longitudinal waves.

Q.6. Which phenomena related to light prove that light is a transverse wave?
Ans: Polarisation is the phenomenon related to light that proves that light is a transverse wave.

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