• Tags: polarization, Malus-law, transverse
• Difficulty: Moderate

Polarization proves that light is a transverse wave (longitudinal waves cannot be polarized). Unpolarized light has electric field vibrations in all directions perpendicular to propagation. A polaroid (or polarizer) transmits only the component along its transmission axis. When unpolarized light of intensity $I_{0}$ passes through a polaroid, the transmitted intensity is $I_{0}$/2. Malus's law: when already-polarized light of intensity I passes through an analyzer at angle θ to the polarization direction, transmitted intensity = Ic$os^{2}$θ. For two crossed polaroids (θ = 90°): I = 0 (no light passes). Inserting a third polaroid between crossed polaroids at angle θ can allow some light through.