Polarization of Light Waves

Introduction to polarization of light waves:

Light is an electromagnetic wave in which electric and magnetic field vectors vary sinusoidally, perpendicular to each other as well as perpendicular to the direction of propagation of wave of light. In common light sources e.g. sun or a bulb, atoms are the elementary radiators of light. As these atoms act independently, light propagated from such sources in a given direction consists of many independent waves whose plane of vibration are randomly oriented about the direction of propagation. Such light waves are said to be unpolarized or polarized randomly.

In principle, each electric field can be can be resolved into y and z components. We can then fond the net electric field along the y-axis and along the z-axis separately. Thus, unpolarized light can be thought of as the superposition of two polarized waves whose plane of vibration are perpendicu;ar to each other. When unpolarized light is passed through a tourmaline crystal cut with its face parallel to its crystallographic axis AB, only those vibrations of light pass through the crystal, which are parallel to AB. All other vibrations are absorbed. That is why intensity of light emerging from the crystal is reduced. The emergent light from the crystal is called plane polarized light. This phenomenon of restricting the vibrations of light in a particular direction, perpendicular to the direction of wave motion is called polarization of light. The tourmaline crystal acts as a polarizer.


Detection of polarized light:


a) A naked eye cannot distinguish between polarized and unpolarized light. A crystal can be used for making this distinction. A calcite crystal, quartz crystal, nicol prism can be used as polarizer as well as analyser of polarized light.

b)When unpolarized light is seen through a single crystal, intensity of transmitted light decreases, on account of polarization. On rotating the crystal, intensity of polarized light does not change.

c)  However, when light transmitted from Polaroid P1is seen through another polaroid P2 and P2 is rotated, the transmitted fraction of light from P2 falls maximum to zero as the angle between P1 and P2 varies from 00 to 900 respectively. Here, P1 is called polarizer and P2is called analyser.


Identification of given light:


For this we pass the given light through Polaroid, rotate the Polaroid about the incident light and examine the emergent light.

i. If there is no change in the intensity of emergent light, incident light is unpolarized.

ii. If there is a change in the intensity of emergent light with minimum not equal to zero, the incident light is partially polarized.

iii. If the intensity of emergent light changes with minimum equal to zero, the incident light is plane polarized.