Polarisation

POLARISATION

The phenomena of interference and diffraction demonstrated that light is propagated in the form of waves. They did not specify whether the light waves are transverse or longitudinal. The phenomena of interference and diffraction are possible in both transverse and longitudinal waves. The phenomenon of polarization distinctly proves that light waves are only transverse in nature. Light is propagated in the form of electromagnetic waves. The phenomenon of restricting the vibrations of light (electric or magnetic field vector) to a particular direction perpendicular to the direction of propagation of wave is called polarization of light. In this lesson only the electric field is considered for discussion.

Plane polarised light

Atransverse wave which has vibrations in all directions in a plane perpendicular to the direction of propagation of wave is said to be unpolarised light as shown in Figure 6.71(a). All these vibrations could be resolved into parallel and perpendicular components as shown in Figure 6.71(b) which represents unpolarised light. If the vibrations of a wave are present in only one direction in a plane perpendicular to the direction of propagation of wave is said to be polarised or plane polarised light as shown in Figure 6.71(c) and 6.71(d).

The plane containing the vibrations of the electric field vector is known as the plane of vibration ABCD as shown in Figure 6.72. The plane perpendicular to the plane of vibration and containing the ray of light is known as the plane of polarisation EFGH.

The Table 6.6 consolidates few characteristics of polarised and unpolarised light.

Polarised light

• Consists of waves having their electric field vibrations in a single plane normal to the direction of ray.

• Asymmetrical about the ray direction

• It is obtained from unpolarised light with the help of polarisers

Unpolarised light

• Consists of waves having their electric field vibrations equally distributed in all directions normal to the direction of ray.

• Symmetrical about the ray direction

• Produced by conventional light sources.