Brewsters Law

Unpolarised light can be polarised by reflection. When unpolarised light is reflected from the surface of an optical material (glass, say) then preferential reflection occurs for the electric-field vector that is perpendicular to plane of incidence. At one particular angle, the polarising angle θ_p, no light at all is reflected apart from those waves whose electric-field vector is perpendicular to the plane of incidence.

Sir David Brewster noticed, in 1812, that when the angle of incidence equals the polarising angle the reflected ray and the refracted are 90^o apart.