As shown in the figure, a ray of unpolarized light enters from water of refractive index into a medium of refractive index after passing through a glass plate of refractive index and a layer of water. At a particular incident angle the reflected ray is polarized in the direction as shown in the figure. The value of (in degrees) is:
Key principle. The reflected ray at the topmost interface is completely polarized when the light hits that interface at Brewster's angle for the water/medium-p boundary. At Brewster's angle the reflected and refracted rays are perpendicular, so the refraction angle into medium p equals .
Snell's law across the stack. For a series of parallel interfaces, . Applying this from the initial water (where the ray makes angle ) to medium p (where the refraction angle is , since the Brewster condition at the top makes the refraction angle in medium p equal to , and the ray in water just below medium p is at , which by parallel interfaces equals ):
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Solve. .
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