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Thermodynamics

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Biot Polariscope

It consists of two glass plates M₁ and M₂. The glass plates are painted black on their back surfaces so as to avoid any reflection and this also helps in absorbing refracted light. A beam of unpolarized light AB is incident at an angle of about 57.5˚C on the first glass surface at B and is reflected along BC. This light is again reflected at 57.5˚ by the second glass plate M₂ placed parallel to the first. The glass plate M₁ as the analyzer.

When the upper plate M₂ is rotated about BC, the intensity of the reflected beam along CD decreases and become zero for 90˚ rotation of M₂. Remember the rotation of plate M₂ about BC keeps the angle of incidence constant and it does not change with the rotation of M₂. Thus we find that light travelling along BC is plane polarized.

When the mirror M₂ is rotated further it is found that the intensity of CD becomes maximum at 180˚, minimum at 270˚ and again maximum at 360˚.

The above experiment proves that when light is incident at an angle of 57.5˚ on a glass surface the reflected light consists of waves in which the displacements are confined to a certain direction at right angles to the ray and we get polarized light by reflection.