8.1 Polarization of Light
Physical optics with reference to polarization deals with the behaviour of light waves and their interaction with matter.
Interference and diffraction effects prove the wave nature of light. These phenomena, however, do not tell us whether the light waves are longitudinal or transverse. Polarization of light suggests that the light waves are transverse in character.
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 vibrations can be confined along vertical, horizontal or any other direction (Fig. 8.1). In each of these cases, the transverse mechanical wave is said to be polarized. 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.
A light wave produced by oscillating charge consists of a periodic variation of the electric field vector accompanied by the magnetic field vector at right angles to it. Ordinary light has components of vibration in all possible planes. Such a light is unpolarized. On the other hand, if the vibrations are confined only in one plane, the light is said to be polarized. Unpolarized light is shown in Fig. 8.2.
Examples of unpolarized light sources are sunlight, incandescent light bulbs, fluorescent light bulbs, light from a candle or fire.
Polarization is the process by which the electric and magnetic vibrations of light waves are restricted to a single plane of vibration. It is the property exhibited only by transverse waves such as light waves. It does not occur for longitudinal waves such as sound waves.