Malus's Law

Malus's Law — choose a source already polarized parallel to P1, or an unpolarized source. P1 transmits all of the first case and one half of the second, so I₁ = I₀ or I₀/2. With two filters the transmission is I = I₁ cos²(P1→P2). With an intermediate P2 it is I = I₁ cos²(P1→P2) cos²(P2→P3).

Record points at the current final-filter angle to compare them with the curve. Crossed P1/P2 filters transmit zero. With crossed outer filters, an intermediate 45° filter gives nonzero transmission. Ideal polarizers and monochromatic intensity factors are assumed; this does not model absorption, wavelength dependence or imperfect extinction.