According to law of Malus, when a beam of completely plane polarised light is incident on an analyser, the resultant intensity of light $(I)$ transmitted from the analyser varies directly as the square of the
cosine of the angle $(\theta)$ between planes of transmission of analyser and polariser.
i.e., $I \propto \cos ^2 \theta$
and $I=I_0 \cos ^2 \theta .......$(i)
where $I_0=$ intensity of the light from polariser.
From equation (i), we note that if the transmission axes of polariserand analyserare parallel(i.e., $\theta=0^{\circ}$ or $180^{\circ}$ ), then $I=I_0$. It means that intensity of transmitted light is maximum. When the transmission axes of polariser and analyser are perpendicular (i.e., $\theta=90^{\circ}$ ), then $I=I \cos ^2 90^{\circ}=0$. It means the intensity of transmitted light is minimum.
On plotting a graph between $I$ and $\theta$ as given by relation (i), we get the curve as shown in figure.
