7.5.2—Malus’s law (I = I0 cos2θ ) to calculate the intensity of a plane-polarised

For plane-polarised light entering an analyser, transmitted intensity is I=I₀ cos²θ, where θ is the angle between the incident polarisation direction and analyser axis.

Use degrees or radians consistently, identify I₀ at θ=0 and remember that an analyser at 90° ideally blocks the light.

At θ=60°, the transmitted intensity is one quarter of I₀ because cos²60°=0.25.

The cosine is squared; using I=I₀cosθ overestimates transmission, and θ is not automatically the angle between two arbitrary filter surfaces.