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.