Q.Unpolarized light with intensity $I_0$ is incident on two polaroids. The axis of the first polaroid makes an angle of 50° with the vertical, and the axis of the second polaroid is horizontal. What is the intensity of the light after it has passed through the second polaroid?

[!TLDR] The angle BETWEEN the two polaroid axes is $90°-50°=40°$ (second axis horizontal, first at 50° from vertical); combining the first-polarizer halving rule with Malus' …

The first polaroid's axis is given at 50° from the VERTICAL; the second polaroid's axis is HORIZONTAL, i.e. at 90° from the vertical. The angle BETWEEN the two polaroid axes -- the angle $\theta$ that Malus' Law actually needs -- is therefore $\theta=90°-50°=40°$, not the raw 50° figure quoted relative to the vertical. …

Compute the angle BETWEEN the two polaroid axes (not the raw angle quoted for the first axis alone) by subtracting from 90° (since the second axis is horizontal), then apply the f …

Using the given 50° directly as the angle $\theta$ in Malus' Law, instead of first computing the actual angle BETWEEN the two polaroid axes (40°, since the second …