The process of confining the vibrations of the electric vector of light to one direction. In unpolarised light, the electric field vibrates in all directions perpendicular to the direction of propagation. After reflection or transmission through certain substances like tourmaline crystal, the electric field is confined to one direction and is said to be plane-polarised light. In circularly-polarised light, the tip of the electric field vector describes a circular helix about the direction of propagation with a frequency equal to the frequency of the light. The magnitude of the vector remains constant. In elliptically-polarised light, the vector also rotates about the direction of propagation but the amplitude changes; a projection of the vector on a plane at right angles to the direction of propagation describes an ellipse. Circularly and elliptically polarised light are produced using a quarter wave plate.
Polarization of light by reflection from the surface of glass was discovered by Malus in 1808. He found that polarized light is obtained when ordinary light is reflected by a plane sheet of glass. Consider the light incident along the path AB on the glass surface ( see figure ). Light is reflected along BC. In the path of BC, place a tourmaline crystal and rotate it slowly. It will be observed that light is completely extinguished only at one particular angle of incidence. This angle of incidence is equal to 57.5 degree for a glass surface and is known as polarizing angle. Similarly polarized light by reflection can be produced from water surface also.
The production of polarized light by glass is explained as follows. The vibrations of the incident light can be resolved into components parallel to the glass surface and perpendicular to the glass surface. Light due to the components parallel to the glass surface is reflected whereas light due to the components perpendicular to the glass surface is transmitted.