POLARIZATION

 In transverse mechanical waves, such as produced in a stretched string, the vibrations of the particles of the medium are perpendicular to the direction of propagation of the waves The vibration can be oriented along vertical, horizontal or any other direction. In each of these cases, the transverse mechanical wave is said to be polarized. 

Plane Polarization 

The plane of polarization is the plane containing the direction of vibration of the particles of the medium and the direction of propagation of the wave. 

Unpolarized Light Waves 

A light wave produced by oscillating charge consists of a periodic variation of electric field vector accompanied by themagnetic field vector at right angle to each other Ordinary light has components of vibration in all possible planes. Such a light is unpolarized. 

Polarized Light Waves 

If the vibrations are confined only in one plane, the light is said to be polarized. 

Production and Detection of Plane Polarized Light

The light emitted by an ordinary incandescent bulb (and also by the Sun) is unpolarized, because its (electrical) vibrations are randomly oriented in space. It is possible to obtain plane polarized beam of light from un-polarized light by removing all waves from the beam except those having vibrations along one particular direction. 

Methods of Polarization 

Polarization can be achieved by various processes such as 

1. selective absorption 
2. reflection from different surfaces
3. refraction through crystals
4. scattering by small particles. 

The selective absorption method is the most common method to obtain plane polarized light by using certain types of materials called dichroic substances. 

Polaroid

The materials transmit only those waves, whose vibrations are parallel to a particular direction and will absorb those waves whose vibrations are in other directions. One such commercial polarizing material is a polaroid.

Experiment to prove that light waves are transverse in nature

1. If un-polarized light is made incident on a sheet of polaroid.
2. The transmitted light will be plane polarized.
3. If a second sheet of polaroid is placed in such a way that the axes of the polaroids, shown by straight lines drawn on them, are parallel, the light is transmitted through the second polaroid also.
4. If the second polaroid is slowly rotated about the beam of light, as axis of rotation, the light emerging out of the second polaroid gets dimmer and dimmer and disappears when the axes become mutually perpendicular.
5. The light reappears on further rotation and becomes brightest when the axes are again parallel to each other. 

Result 

This experiment proves that light waves are transverse waves. If the light waves were longitudinal, they would never disappear even if the two polaroids were mutually perpendicular

Reflection from different surfaces 

Reflection of light from water, glass, snow and rough road surfaces, for larger angles of incidences, produces glare. Since the reflected light is partially polarized, glare can considerably be reduced by using polaroid sunglasses. 

Reflection by small particles 

Sunlight also becomes partially polorized because of scattering by air molecules of the Earth's atmosphere. This effect can be observed by looking directly up through a pair of sunglasses made of polarizing glass. At certain orientations of the lenses, less light passes through than at others.

Optical Rotation

When a plane polarized light is passed through certain crystals, they rotate the plane of polarization. Quartz and sodium chlorate crystals are typical examples, which are termed as optically active crystals. 

Refraction through crystals

A few millimeter thickness of such crystals will rotate the plane of polarization by many degrees. Certain organic substances, such as sugar and tartaric acid, show optical rotation when they are in solution. This property of optically active substances can be used to determine their concentration in the solutions.