Q.Unpolarized light of intensity $I_i$ is incident on a polaroid. The plane polarized light transmitted by this polaroid then falls on a second polaroid (analyzer) whose transmission axis makes an angle of $60^\circ$ with that of the first. Calculate the intensity of the light emerging from the analyzer, in terms of $I_i$.

The first polaroid converts the incident unpolarized light of intensity $I_i$ into plane polarized light of intensity exactly half the incident value:

This polarized light of intensity $I_0$ then passes through the analyzer, whose transmission axis is at $\theta=60^\circ$ to the first polaroid's axis. By Malus's law, …

First halve the incident intensity for the polarizer, then apply Malus's law $I=I_0\cos^2\the …

• Applying Malus's law directly to the ORIGINAL unpolarized intensity $I_i$ instead of to the already-halved polarized intensity $I_0=I_i/2$. …