What is Malus law ?

When a completely plane-polarised light beam is incident on an analyser, the intensity of the emergent light varies as the square of the cosine of the angle between the planes of transmission of the analyser and polarizer. This is known as Malus law. $\mathrm{I}={\mathrm{I}}_0{\cos }^2\mathrm{\theta }$
I is the intensity of light emerging from the analyser. I₀ is the intensity of the incident-plane polarized light.