Polarization Of Light
Light is an electromagnetic wave in which electric and magnetic field vectors vary sinusiodally, perpendicular to each other as well as perpendicular to the direction of propagation of wave light.
The phenomena of polarization of light can be demonstrated experimentally by a simple arrangement; hereT1andT2are two thin plates of tourmaline, cut with their faces parallel to the axis of the crystal. A fine pencil of ordinary light from the source S is passed through the plateT1; and the light transmitted is observed by the naked eye.
When the plateT1is rotated about the direction of propagation of light as axis, the intensity and character of transmitted light remain the same (except for slight change in color due to selective absorption within the crystal). Let the second plateT2be placed in the path of light transmitted fromT1. We observe that intensity and character of light transmitted only whenT1andT2are set with their axes parallel. WhenT2is rotated gradually, intensity of light transmitted fromT2goes on decreasing. As soon as the axes of the two crystals are at90°to each other, light is completely cut off. On rotatingT2further, light reappears. Intensity of light transmitted fromT2starts increasing, till is maximum when axes of the two crystals are parallel again.
If bothT1andT2are rotated with the same angular velocity in the same direction, no change in intensity of light is observed.
The phenomenon can be explained when we assume that light waves are transverse. Now the light falling onT1has transverse vibrations of electric vector lying in all possible directions. The crystalT1allows only those vibrations to pass through it, which are parallel to its axis. When the crystalT2is, introduced with its axis kept parallel to the axis ofT1, the vibrations of electric vector transmitted byT1are also transmitted throughT2.
However, when axis ofT2is perpendicular to axis ofT1, vibrations of electric vector transmitted fromT1are normal to the axis ofT2. Therefore,T2does not allow them to pass and hence eye receives no light.
Light coming out of the crystalT1is said to be polarized i.e. it has vibrations of electric vector which are restricted only in one direction (i.e. parallel to the optic axis of crystalT1).
Since the intensity of polarized light on passing through a tourmaline crystal changes, with the relative orientation of its crystallographic axes with that polarizer, therefore, light must consist of transverse waves.
Detection of polarized light
A naked eye cannot distinguish between polarized and unpolarized light. A crystal can be used for making this distinction. A calcite crystal, quartz crystal, a nicol prism (made from calcite crystal) can be used as polarizer as well as analyzer of polarized light.
当偏振光通过一个克丽丝tal (Polaroid), the intensity of transmitted light decreases on account of polarization. On rotating the crystal, intensity of polarized light does not change.
However when light transmitted from PolaroidP1is seen through another PolaroidP2andP2is rotated, the transmitted fraction of light fromP2falls from maximum to zero as the angle betweenP1andP2varies from0° to 90°respectively. Here,P1is called polarizer andP2is called analyzer.
Identification of given light beam
For this, we pass the given light through a Polaroid (called analyzer), rotate the Polaroid about the incident light and examine the emergent light.
(i)If there is no change in intensity of emergent light, incident light is unpolarized.
(ii)If there is change in intensity of emergent light with minimum not equal to zero, the incident light is partially polarized.
(iii)如果intensity of emergent light changes with minimum equal to zero, the incident light is plane polarized or linearly polarized.
When angles between the principle sections of two nicols are0°and180°, they are referred to as parallel nicols. When this angle is90°, they are said to form crossed nicols.
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