Polarisation:

Ordinary radiation is created through vibration of electric charges within all possible directions. Therefore a radiation consists of a bundle of electromagnetic waves in that the vibrations are equally distributed between an infinite series of planes perpendicular to the direction of propagation. Such a radiation that is vibrating in more than one plane is referred to as unpolarised radiation. Here if we observe an approaching unpolarised radiation from the direction opposite to its direction of propagation we would observe a set of infinite electrical vectors that oscillate around a fixed point as display in Figure.

Figure: An unpolarised radiation passing through a polariser to give a plane polarised light

While such an unpolarised light is passed by a polariser, it cuts off all the radiations oscillating in different planes; and permits only those rays which vibrate in any one particular plane to pass by. The emerging radiation is known as the plane polarised light. A electric and magnetic elements of the plane polarized light vibrate at right angles to each other as shown in above figure. The procedure of transforming unpolarised light within polarised light is known as polarisation. Calcite or quartz crystals are generally used to acquire the polarised radiations.