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If a ray of light is incident at an interface between two media in such a manner that the reflected and transmitted rays are at right angles to each other, the angle of incidence, $$B$$, is called the Brewster angle.
A moment’s thought will show that, if the refractive indices are $$n_1$$ and $$n_2$$. $$\tan B = n_2 / n_1$$ . For example, at an air $$(n_1 = 1)$$ to glass $$(n_2 = 1.5)$$ interface the Brewster angle is 56 degrees.