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Question 4: How you can explain Brewster's law of polarization?


Light is electromagnetic waves that can travel through vacuum. These waves are produced by electric and magnetic field vibrations perpendicular to one another. These vibrations occur in numerous planes in space. Such light waves in which vibrations of electric and magnetic fields occur in many (more than one) planes are called non-polarized light. However, the electric and magnetic fields of light can be restricted to one plane, which is then called polarized light. The unpolarized light can be polarized in many ways like;

(1) Use of Polaroid filter.
(2) Polarization by reflection.
(3) Polarization by refraction.
(4) Polarization by scattering


Brewster proposed his law in 1811. It tells us about the relation of the refractive indexes to the polarizing angle in case when light is reflected from the non-metallic surfaces like glass, water, roads, snowfields etc.

It states when unpolarized light is incident at an angle called Brewster’s angle or angle of polarization, the light that is reflected is perfectly polarized. The relation of angle of incidence and the refractive indexes is given by the relation; 

Proof of Brewster's law

Consider an unpolarized light (from medium 1) incident on a transparent surface (medium 2). In the diagram, the red line is the incident unpolarized ray. Suppose n1 and n2 are the respective refractive indexes of the two mediums. The reflected light ray (green ray in the diagram) is generally partially plane polarized. However, if we slowly change the direction of the incident ray, at a certain angle ip the reflected ray will be completely polarized. This angle is called polarization angle or Brewster’s angle. At this angle, the reflected ray and the refracted ray (yellow ray in the diagram) are found to be at right angle to each other. Also, the vibrations in the polarized rays are parallel to the surface of medium 2.

Now applying Snell’s law,

From the fig;

Now putting this value in equation (2),

This equation is called Brewster’s law.


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