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; hereT1 and T2 are 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 plate T1; and the light transmitted is observed by the naked eye.
When the plate T1 is 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 plate T2 be placed in the path of light transmitted from T1. We observe that intensity and character of light transmitted only when T1 and T2 are set with their axes parallel. When T2 is rotated gradually, intensity of light transmitted from T2 goes on decreasing. As soon as the axes of the two crystals are at 90° to each other, light is completely cut off. On rotating T2 further, light reappears. Intensity of light transmitted from T2 starts increasing, till is maximum when axes of the two crystals are parallel again.
If both T1 and T2 are 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 on T1 has transverse vibrations of electric vector lying in all possible directions. The crystal T1 allows only those vibrations to pass through it, which are parallel to its axis. When the crystal T2 is, introduced with its axis kept parallel to the axis of T1, the vibrations of electric vector transmitted by T1 are also transmitted through T2.
However, when axis of T2 is perpendicular to axis of T1, vibrations of electric vector transmitted from T1 are normal to the axis of T2. Therefore, T2 does not allow them to pass and hence eye receives no light.
Light coming out of the crystal T1 is 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 crystal T1).
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.
When polarized light is seen through a single crystal (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 Polaroid P1 is seen through another Polaroid P2 and P2 is rotated, the transmitted fraction of light from P2 falls from maximum to zero as the angle between P1 and P2 varies from 0° to 90° respectively. Here, P1 is called polarizer and P2 is 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) If 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 are 0° and 180°, they are referred to as parallel nicols. When this angle is 90°, they are said to form crossed nicols.
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