Polarization of Light - Definition, Types & Applications

Polarization of Light is a property applying to transverse waves that specify the geometrical orientation of the oscillations. Light is an electromagnetic wave in which electric and magnetic field vectors are sinusoidally perpendicular to each other as well as they are perpendicular to the direction of propagation of light waves.

When the electric field vectors are restricted to a single plane by filtration, then the light is said to be polarized with respect to the direction of propagation and it propagates in a single plane. In this article, we are going to learn about the polarization of light, brewster’s law, double refraction, polarization by scattering. Let’s understand the Physics (Optics) phenomenon of unpolarized light first.

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Unpolarized Light:

When light wave propagates it consists of vibration of electric field in all possible direction, which are perpendicular to its direction of propagation and such light waves are said to be unpolarized light.

Polarization is nothing but restriction of the motion of propagation of light.

Polarization of Light:

When we restrict vibration to an electrical vector in an unpolarized light we get plane polarized light. In this the direction of vibration of electric vector and direction of light propagation are perpendicular to each other as shown below.

• When we restrict vibration to an electrical vector in an unpolarized light we get plane-polarized light.
• In this the direction of vibration of electric vector and direction of light propagation are perpendicular to each other as shown below.

Types of Polarization:

There are three main types of polarization namely, linear polarization, circular polarization, and elliptical polarization.

1. Linear polarization is when the oscillation of a wave is constrained to a single plane. This is the plane of polarization.

2. Circular polarization is when two linear components of a wave oscillate perpendicular to each other such that their amplitudes are equal.

3. Elliptical polarization is the same as circular polarization except that the amplitudes and phase differences are not the same.

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Terminology of Polarization:

• Plane of vibration: The plane in which vibration of electric field vectors are restricted are termed as plane of vibration.

The figure represents the motion of plane polarized light in both vertical and horizontal direction respectively.

• Plane of polarization: The plane which is perpendicular to the plane of vibration i.e., the plane in which there is no vibration of electric field vectors are known as planes of polarization.
• Polarizers: The device which is used to polarize an unpolarized light are known as polarizers and some of its examples are Nicol prism, Tourmaline crystal, etc.
• Analyzer: The device which is used to determine the plane of polarization is termed as analyzer.
• Polaroids: It is a large sheet made up of microscopic dichroic crystals which can produce a beam of polarized light.

Experiment to Determine Transverse Nature of Light:

For this experiment let’s consider that we have an unpolarized light passing through two polarizer T1 and T2, here line on polarizer shows plane of vibration i.e., the plane in which electric field vector can vibrate freely, whereas electric field perpendicular to this line will be eliminated completely.

Now as this unpolarized light passes through the polarizer, it will polarize the light in either vertical or horizontal direction. This polarized light when again passed through a polarizer there can be two possible outcomes.

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• Case I

If the plane of vibration of both polarizers are parallel to each other in such polarized light from the first polarizer can easily pass through the second polarizer as shown below.

• Case II

Now if we rotate second polarizer in such a way that it becomes perpendicular to first one, in this case as plane polarized light passes through second polarizer it will get eliminated since second polarizer won’t allow it to pass and hence it is called analyzer which is used to determine the plane of polarization as shown below

And hence from this experiment we can conclude that electric fields are vibrating perpendicular to its direction of propagation, hence light is a transverse wave.

Brewster’s Law (Polarization by Reflection):

• Brewster’s law states that the tangent polarized angle is equal to the refractive index of the refracting medium at which partial reflection of light took place.
• According to Brewster’s law when an unpolarized monochromatic beam of light is incident on a plane refracting surface, part of light will be reflected and rest will be refracted as shown below
• In this we can see that the reflected light will be completely polarized in the plane of incidence at a certain angle of incidence, hence this angle is known as polarizing angle.
• Whereas at this angle the reflected and refracted ray are separated by an angle of 90° from each other.
• Now if we consider ip the polarizing angle, rp be refracted angle and μ as the refractive index of the medium then from the figure we can see that

And Now by using Snell’s law

Check Uniform Circular Motion article here.

Thus using Brewster’s law, we can conclude that polarizing angle depends on frequency of incident ray i.e., colour of incident ray hence polarizing angle will be different for different colour.

Example of Brewster’s Law:

Q: What will be the polarizing angle of a fluid with a refractive index of 1.5 with respect to air?

Ans: Given:

Refractive index of fluid with respect to air, n = 1.50

Now according to Brewster’s law angle of polarization is given as

Double Refraction (Polarization by Refraction):

Double refraction, also known as birefringence, is an optical property in which a single ray of unpolarized light entering an anisotropic medium is split into two rays i.e., ordinary & extraordinary rays, each traveling in a different direction.The concept of Double refraction can be observed by comparing two materials, glass and calcite.

Suppose if a pencil mark is drawn upon a sheet of paper and then covered with a piece of glass, only one image will be seen. But if the same paper is covered with a piece of calcite and the crystal is oriented in a specific direction then two marks will become visible. This is because a refracted beam acquires some degree of polarization. Most often, the polarization occurs in a plane perpendicular to the surface.

Light passing through a calcite crystal is split into two rays as shown below. This process is called double refraction or polarization by refraction.

Get the Eddy Current & Displacement Current in detail.

The two rays of light are each plane-polarized by the calcite such that the planes of its polarization are mutually perpendicular to each other as shown below.

For calcite, one of the two rays does indeed obey Snell’s Law (angle of incidence is equal to the angle of refraction), this ray is called the ordinary ray (or O-ray). The other ray (and any ray that does not obey Snell’s Law) is an extraordinary ray (or E-ray). For normal incidence (a Snell’s law angle of 0°), the two planes of polarization are also perpendicular to the plane of incidence.

The velocity and refractive index of extraordinary waves changes with direction whereas for ordinary ray velocity remains constant throughout the crystal while propagating.

Some other differences between o-ray and e-ray are as shown below

For monoclinic or tetragonal crystals, we get the above result i.e., One O-ray and one e-ray can be observed. Whereas for orthorhombic, monoclinic, and triclinic crystals, there will be two E-rays

Check Power in AC Circuit for details here.

Optical Activity and Optical Rotation:

Optical activity is the ability of a substance to rotate the plane of polarization of a beam of light that passed through it.

A polarimeter is an instrument used to determine the angle through which plane-polarized light has been rotated by a given sample.

The optical rotation is the angle through which the plane of polarization is rotated when polarized light is passed through a layer of a liquid.

Substances are described as dextrorotatory or levorotatory according to whether the plane of polarization is rotated clockwise or counterclockwise, respectively, as determined by viewing towards the light source. Dextrorotation is designated (+) and levorotation is designated (-).

The specific rotation is a characteristic property of a certain substance and is the standard measurement for the optical rotation of that substance.

The optical rotation and specific rotation are related by

where,

= optical rotation

c = the concentration (g/dl)

l = path length (dm)

Read about the Atom & Nuclei here.

Example of Optical Rotation:

Q: A sugar solution in a tube of length 2.0 dm produces optical rotation of 12°. Then, the sugar solution to one half of its initial concentration. If the dilute solute solution is contained in another tube of length 3.0 dm, the optical rotation produced by it will be

A: Given

when l = 2 dm and concentration c;

Now the concentration is c/2 and l = 3 dm

Polarization by Scattering:

So, as we discussed above, light is a transverse wave, and confining the plane of vibration of such a transverse wave by the principle of scattering is termed Polarization by scattering.

Also, we know that when an unpolarized light passes through a medium, it undergoes a phenomenon known as scattering.

Such a medium contains lots of atoms and when an unpolarized light beam strikes an atom it transfers energy to an atom. As a result, an electron in that atom starts vibrating in response to the electric component of the incident ray. Now, this vibrating electron emits E.M radiation in all directions, and this forces neighboring electrons to undergo a similar process. This phenomenon of absorption and emission of the electromagnetic radiation is known as the scattering of light.

However the scattered light can be unpolarized, partially polarized, or completely polarized based on the direction of the incident ray as shown below.

Polarization by scattering is generally observed in liquid or gas randomness in motion of atoms. However, in perfect crystals we don’t see as good results as fluids.

Polarization due to scattering can be seen through the sky as we can see below. When an unpolarized beam of sunlight passes through the atmosphere the light scatters down towards earth. Also it has no vertical polarization.

This is the reason the sky scatters more blue light than red because of the natural frequency of molecules in our atmosphere.

We are sure that the above Physics (Optics) notes helped you to understand the importance of Polarization of Light. Get more study notes, practice sets, and puzzles, etc. on the Testbook App. Download the app now to avail exciting offers on select subscriptions!

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