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When white light (unpolarized light) beam is reflected from a surface, the reflected light could be, unpolarized, partially polarized, or completely polarized depending on the angle of incidence. For a particular angle of incidence, the reflected light is completely polarized.
Let an unpolarized light beam is incident on a surface, as shown in above figure. Now suppose that the angle of incidence θ is varied until the angle between the reflected and refracted beams is 90°. At this particular angle of incidence, the reflected beam is completely polarized and the refracted beam is still only partially polarized. The angle of incidence at which this polarization occurs is called the polarizing angle θp.
We can see from the figure  i.e. . Using Snell’s law of refraction;
So, 
The above expression is called Brewster’s law, and the polarizing angle θp is called Brewster’s angle.
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Waves and Optics