Statement: When unpolarized light hits a surface at the Brewster angle $\theta_p$, the reflected beam is completely plane-polarized (perpendicular to the plane of incidence).

Brewster's law: $\tan\theta_p = n$ where $n$ = refractive index of the second medium.

Geometric condition: At Brewster's angle, reflected and refracted rays are perpendicular: $\theta_p + \theta_r = 90°$

Derivation: By Snell's law: $\sin\theta_p = n\sin\theta_r = n\sin(90°-\theta_p) = n\cos\theta_p$. Therefore $\tan\theta_p = n$.

Typical values:

Application: Polaroid sunglasses block glare from horizontal surfaces (water, roads) by having a vertical transmission axis — exploiting that reflected glare is horizontally polarized (Brewster's angle reflection from surfaces).