Polarisation

The phenomena of interference, diffraction, reflection and refraction are characteristics of all waves, either of transverse or of longitudinal. However, the polarization is the characteristics of the transverse wave.

Polarization

An ordinary light beam consists of a large number of waves emitted by the atoms of a light source. Each atom produces a wave having some particular orientation of the electric field vector E as shown in a figure. The magnetic field B associated with the wave is perpendicular to the field E and both are perpendicular to the direction of propagation.

The direction of polarization of each wave is defined by the direction in which the electric field E is vibrating. In the figure, direction lies along the y-axis. However, an electromagnetic wave could its E vector in the yz-plane making an angle with the y-axis. Because all directions of vibration from a wave source are possible, the resultant electromagnetic wave is a superposition of waves vibrating in many different directions. This gives an unpolarized light beams as shown in the figure, which is propagating in the direction of vibration of the electric field vectors for the individual waves making up the resultant wave.

A wave is linearly polarized if the resultant electric field E vibrates0 in the same direction at all time at a particular point as shown in the figure. The plane formed by E and the direction of propagation is called the plane of polarization of the wave. If the wave represents the resultant of all individual waves, the plane of polarization is xy-plane. A linearly polarized beam can be obtained from an unpolarized beam by removing all waves from the beam except those whose electric field vectors vibrate in a single plane.

Polaroid

Polaroid is a device used to produce a plane polarized light. It consists of a long chain molecule aligned in a particular direction. Those crystals which are used to make Polaroid are crystals of iodosulphide of quinine in thin sheet mounted between two thin sheets of glass or cellulose. These crystals are also called herapathide.

When two polaroids are parallel to each other, the polarizing direction is parallel to each other and the light transmitted by the first Polaroid is also transmitted by the second as shown in the figure. When two polaroids cross each other, i.e. their transmission axis is mutually perpendicular to each other than the light transmitted through first Polaroid is completely blocked by the second polaroid as shown in the figure. Polaroid has a variety of uses in a daily life.

1. The light reflected from the bright surfaces, wet roads, and snow produces glare while reaching the eye. This glare, harmful to an eye, can be avoided by wearing sunglasses made of polaroids with the vertical transmission axis. A sunglass polarizes the sun rays.
2. Polaroids are used as glass windows in trains and aeroplanes. One of the polaroids is fixed and while the other can be rotated to have the desired intensity of light.
3. Polaroids are used to eliminate the dazzle from the headlights of cars, buses and other vehicles.
4. Polaroids are used to record and reduce three-dimensional moving pictures.
5. Polaroids are used in photoelastic stress analysis or photo-elasticity.
6. Liquid crystal display (LCD) in calculators, watches etc. use the principle of polarization to form numbers and letters.

Reference

Manu Kumar Khatry, Manoj Kumar Thapa, Bhesha Raj Adhikari, Arjun Kumar Gautam, Parashu Ram Poudel. Principle of Physics. Kathmandu: Ayam publication PVT LTD, 2010.

S.K. Gautam, J.M. Pradhan. A text Book of Physics. Kathmandu: Surya Publication, 2003.