Malus's Law
Definition
How polarizing filters dim light — two crossed polarizers block everything; at an angle θ the intensity drops as cos²θ.
Technical definition
I = I₀ cos²θ for already-polarized light passing through an analyzer at angle θ to the polarization direction.
Example
Hold polarized sunglasses against another polarizer and rotate 90° — the world goes dark, because cos²(90°) = 0.
Common confusion
Malus's law applies to already-polarized light. Unpolarized light through a single polarizer always drops to half (I₀/2) regardless of angle — Malus takes over at the second filter.