Unpolarised light of intensity $I_0$ is passed through a single ideal polariser. The intensity of the transmitted light is:

Unpolarised light contains all polarisation directions with equal probability. Averaging Malus's law over all input angles gives $\langle I \rangle = I_0 \langle\cos^2\theta\rangle = I_0/2$. Once polarised, a second polariser would then attenuate further by $\cos^2\theta$ relative to its axis.