What is the maximum intensity of light transmitted through three polaroids when the first and third are crossed, and the
**Polaroid rotation** intensity varies sinusoidally with angle due to Malus law I = I₀ cos²θ, when polaroid rotated 90° from initial, intensity goes from max to zero, for unpolarized light rotating polaroid does not change intensity after first polaroid because average, but second polaroid intensity depends on relative angle, explains why intensity transmitted through two polaroids drops to zero when perpendicular. For crossed polaroids, intensity after the second polaroid is I₀ cos² θ , and after the third (at 90° - θ ) is I = I₀ cos² θ sin² θ = (I₀/4) sin² 2θ . Maximum occurs at θ = 45° , so
Ref: NCERT > Physics Book > Wave Optics > Polarization and Malus Law