Polarization

Polarization

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Polarization

We know that light is an electromagnetic wave. Light waves are transverse waves. The electric field of the light wave oscillates perpendicular to the direction of propagation. Light wave which vibrates in more than one plane is called An Unpolarized Light. Examples for unpolarized light is Sun light, LED spotlights, incandescent bulbs, light which is emitted by a candle flame. These light waves are produced by electric charges and vibrate in different directions. This unpolarized light is capable of transforming into polarized light. Polarized light waves vibrates only in a single plane. This process of transforming unpolarized light into polarized light is called Polarization. In polarization, the electric field off light is well defined. Brewster windows, absorbing thin film reflectors use the concept of Polarization.

PolarizationPolarization

There are different types of polarization depending on the orientation of electric field. Circular Polarization, which has constant amplitude and a phase difference of π/2. In this case the orientation describes a circle. Linear polarization has constant amplitude and same orientation. Elliptical polarization has orientation varying with time. Here the electrical field of describes an ellipse. The electrical field of elliptically polarized light has two perpendicular linear components with any amplitude and phase difference.

Types of polarizationTypes of polarization

Methods of Polarizing light

There are mainly four methods of polarizing light. They are polarization by reflection, polarization by refraction, polarization by scattering, polarization by transmission.

Polarization by reflection

Unpolarized light will undergo polarization by the reflection of nonmetallic surfaces. The amount of polarization depends on the angle of light proceeding towards the surface and also the surface by which it is made up of. Metallic surfaces reflect light with a variety of vibrational directions which are unpolarized ones. Nonmetallic surfaces and water reflect light where there will be large concentration of vibrations in a plane parallel to the reflecting surface.

Polarization by refraction

Consider a unique crystal like calcite that act as a double refracting crystal. It refracts the incoming light or the unpolarized into two different paths. Thus double refraction takes place. The one light beam obeys the laws of refraction and the other one does not obey the Snell’s law of refraction. Now if we view an object through the crystal, we can see two images which are the result of double refraction of light. It is due to the chemical structure and specific characteristics of these crystals. Here one beam is partially polarized and the other one is completely polarized. Both the beams are polarized in which one is in the direction parallel to the surface and the other one in the direction perpendicular to the surface. Now we need a polarizing filter to block one of the two images and thus we get one image after the process of polarization by refraction.

Polarization by scattering

When light is incident on an atom or molecule then a part of the incident light is absorbed and it is re- radiated in various directions. The re radiated light from the atom or molecule is called Scattering of Light. The scattered light which is in the direction perpendicular to the incident light is completely plane polarized and the transmitted light is partially polarized. The transmitted light is the one which is in the direction of incident light. The light parallel to the original ray has no polarization.

Polaroid

A polaroid is the material which is used to polarize light. The principle used for polaroids are of selective absorption. It is able to polarize light due to the chemical composition of the filter material. It consists of micro crystals of herapathite and each crystal is a double refracting medium. Recently, new types of these polaroids are introduced in which thin film of polyvinyl alcohol is used. They are colorless crystals and can transmit more light and results in better polarization. Polaroids are widely used in polarizing sunglasses. It is used to improve color contrasts in oil painting. It is used to eliminate the head light glare in motor cars.

Application of PolaroidApplication of Polaroid

Brewster’s Law

Sir David Brewster proposed Brewster’s law. When an unpolarized light is incident at an angle which is angle of polarization or brewster’s angle, the reflected light is completely polarized. It states that the tangent of the angle of polarization is equal to the refractive index of the respective medium. µ = tan ip, where µ is the refractive index and ip is the angle of polarization. Brewster’s angle, also known as polarizing angle is the angle at which incident unpolarized light is reflected after complete polarization. Brewster’s angle is calculated by using the equation µ = Sin (ip) / Sin (r). Here ip is the angle of incidence and r is the angle of refraction.

We can write r = 90° – ip

From Snell’s law µ = Sin ip / Sin r

Substitute for r and then Sin r = Sin (90°- ip) = Cos ip

µ = Sin ip / Sin (90° - ip)

So Sin ip / Cos ip = µ

µ = tan ip

Thus it proves the Brewster’s law.

Again Cos ip = Sin r = cos (π/2-r)

Therefore ip = π/2- r

i + r = π/2, Here the reflected ray and refracted rays are at right angles to each other.

Brewster’s LawBrewster’s Law

Malus’s Law

The law states that after the light is incident on the analyzer, the intensity of the polarized light which is transmitted by the analyzer is directly proportional to the square of the cosine of the angle between the transmission axes of the analyzer and polarizer.

I0  A02, where A0 is the amplitude of the electric vector which is transmitted by the polarizer. IO is the intensity of the light which is incident on the surface. The analyzer only transmits A0 Cos θ of the component and A0 sin Ɵ of the component is absorbed by the analyzer.

I  (A0 Cos θ)2.

I / I0 = (A0 Cos θ)2/ A02 = Cos θ.

I = I0 Cos θ. Thus I∝ Cos θ, which proves the malus law.

Malus ‘s law

Malus ‘s law

Applications of Polarization

Polarization id used for 3D movies. Polarized 3D glasses with polarized filters makes each eye receiving only the calculated image. It is possible to get rid of the glare and hot spots and bring out the surface defects. Polarized light is used photo elastic stress analysis. It is useful in various fields of engineering and industry area. It is the used in the field of communication and radar application. Polarization is used in infrared spectroscopy.

Summary

  • Polarized light waves are the ones in which the vibrations occur in a single plane.
  • The different types of polarization are circular, linear and elliptical polarization.
  • Polarizations are possible by different methods like polarization by reflection, refraction and scattering.
  • Polaroid is used to polarize light.
  • Brewster’s law states that µ = tan ip, where µ is the refractive index and ip is the angle of polarization
  • I ∝ Cos θ, is the malus law here I is the intensity of polarized light transmitted by the analyzer.
  • Polarization is used in a variety of applications

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Polarization

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