Polarization Of Light

Polarization

A light wave is an electromagnetic wave that travels through the vacuum of outer space. Light waves are produced by vibrating electric charges. For our purposes, it is sufficient to merely say that an electromagnetic wave is a transverse wave that has both an electric and a magnetic component.TRANSVERSE NATURE OF WAVE

The transverse nature of an electromagnetic wave is quite different from any other type of wave. Let's suppose 1 | P a g e

that we use the customary slinky to model the behavior of an electromagnetic wave. As an electromagnetic wave traveled towards you, then you would observe the vibrations of the slinky occurring in more than one plane of vibration. This is quite different than what you might notice if you were to look along a slinky and observe a slinky wave traveling towards you. Indeed, the coils of the slinky would be vibrating back and forth as the slinky approached; yet these vibrations would occur in a single plane of space. That is, the coils of the slinky might vibrate up and down or left and right. Yet regardless of their direction of vibration, they would be moving along the same linear direction as you sighted along the slinky. If a slinky wave were an electromagnetic wave, then the vibrations of the slinky would occur in multiple planes. Unlike a usual slinky wave, the electric and magnetic vibrations of an electromagnetic wave occur in numerous planes. A light wave that is vibrating in more than one plane is referred to as unpolarized light.

Polarization of Light:

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Light is a form of energy which enables us to see the objects from which it comes.

Light waves are electromagnetic in nature can travel with a velocity of 3 x 108 m/sec. in free space.

Light waves show a number of properties such as Interference, Diffraction, Polarzation, Doppler Effect, Reflection, Refraction and Dispersion etc.

Polarization of light is one of the unique and most significant properties shown by light waves.

Light emitted by the sun, by a lamp in the classroom, or by a candle flame is unpolarized light. Such light waves are created by electric charges that vibrate in a variety of directions, thus creating an electromagnetic wave that vibrates in a variety of directions. This concept of unpolarized light is rather difficult to visualize. In general, it is helpful to picture unpolarized light as a wave that has an average of half its vibrations in a horizontal plane and half of its vibrations in a vertical plane.It is possible to transform unpolarized light into polarized light. Polarized light waves are light waves in which the vibrations occur in a single plane. The process of transforming unpolarized light into polarized light is known as polarization. There are a

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variety of methods of polarizing light. The four methods discussed on this page are:

Polarization by Transmission Polarization by Reflection Polarization by Refraction Polarization by Scattering Polarization by Dichroism

Polarization by ReflectionUnpolarized light can also undergo polarization by reflection off of nonmetallic surfaces. The extent to which polarization occurs is dependent upon the angle at which the light approaches the surface and upon the material that the surface is made of. Metallic surfaces reflect light with a variety of vibrational directions; such reflected light is unpolarized.

  

Polarization by Refraction

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Polarization can also occur by the refraction of light. Refraction occurs when a beam of light passes from one material into another material. At the surface of the two materials, the path of the beam changes its direction. The refracted beam acquires some degree of polarization.

Polarization by Scattering

When a beam of white light is pass through a medium containing particles whose size is of the order of wavelength of light. Then the beam gets scattered. When the scattered light is seen in a direction perpendicular to the direction of incidence, it is found to be plane polarized. This is called polarization by scattering.

Polarization by Dichroism

Some doubly- refracting crystals have the property of absorbing strongly one of the two refracted rays,

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while allowing the other to emerge with little loss. This selective absorption by the crystals is known as Dichroism. The best example of such a crystal is Tourmaline.

Polarization by Use of a Polaroid Filter

Information about Polaroids

Polaroids - Polaroid is based on the phenomenon of dichroism. It is a big polarising film mounted between two glass plates and is used to obtain plane Polarized light. To prepare this film, ultra-microscopic crystals of herpathite (lodosulphate of Quinine) are spread by a special device in a thin sheet of nitro-cellulose in such a way that the optic axes of all the crystals are parallel. Hence, when a beam of unpolarized light passes through the polaroid film, the emerging light is plane-Polarized.

Uses of Polaroids -

a) Polaroids are used in the laboratory to produce and analyse Plane Polarized light.

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b) They are used in sun glasses to cut off the light reflected from horizontal surfaces such as wet roads, polished tables etc.

c) Polaroids are used in motor cars to avoid the dazzling light of a car approaching from opposite directions.

d) Polaroid glasses are used for viewing three dimensional pictures.

e) Polaroids are used in photographic camera as filters. These filters eliminate the glare of the reflected light.

Malus Law

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When a completely plane polarized light beam is incident on an analyser, the intensity of the emergent light varies as the square of the cosine of the angle between axes of polariser and analyser. It is known as Malus Law.

i. e I µ Cos2 q

Or, I = I0 Cos2 q

If q = 0 i.e. the polarizer and analyser are parallel

Then I = I0 (Maximum)

And if q = 900 i.e. the polariser and analyser are perpendicular.

Then I = 0 (Minimum)

To Detect Plane Polarized Light

When unpolarized light is seen through a single crystal (polaroid) intensity of transmitted light decreases, on account of Polarization, on rotating crystal, intensity of Polarized light doesn't change.

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Actually a combination of polariser and analyser helps in detection of plane Polarized light.

When unpolarized light is transmitted through a Polaroid filter, it emerges with one-half the intensity and with vibrations in a single plane; it emerges as polarized light.

 

A Polaroid filter is able to polarize light because of the chemical composition of the filter material. The filter can be thought of as having long-chain molecules that are aligned within the filter in the same direction. During the fabrication of the filter, the long-chain molecules are stretched across the filter so that each molecule is (as much as possible) aligned in say the vertical direction. As unpolarized light strikes the filter, the portion of the waves vibrating in the vertical direction are absorbed by the filter. The general rule is that the electromagnetic vibrations that are in a 9 | P a g e

direction parallel to the alignment of the molecules are absorbed.

Optical Activity

There are certain substances which have property to rotate the plane Polarized light's plane of vibration when passed through them, this is called optical activity. Such substances are called optically active substances.

Two Categories of Substances:

(i) Dextrorotatory (Right Handed)

Substances that rotate light in clock wise direction are called dextrorotatory.

(ii) Laevorotatory (Left Handed)

Substances that rotate light in anti-clock wise direction are called laevorotatory.

Dependence of Angle of Rotation:

(i) Thickness of crystal.

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(ii) Density of crystal.

(iii) Wavelength of light used.

(iv) Temperature of the solution.

Applications of PolarizationPolarization has a wealth of other applications besides their use in glare-reducing sunglasses. In Industry, Polaroid filters are used to perform stress analysis tests on transparent plastics. As light passes through a plastic, each color of visible light is polarized with its own orientation. If such a plastic is placed between two polarizing plates, a colorful pattern is revealed. As the top plate is turned, the color pattern changes as new colors become blocked and the formerly blocked colors are transmitted. Polarization is also used in the entertainment industry to produce and show 3-D movies.

BIBLIOGRAPHY

1. HELP FROM TEACHERS

2. INTERNET.

3. www.google.com

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4. N.C.E.R.T Textbooks

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