Unpolarised light of intensity 4.0Wm^−2 is incident on a polariser placed at 15∘ to the vertical.
State the intensity of the transmitted light.
I've tried 4 x cos(15)^2 = 3.732050808, but it says "Unpolarised light is a random mix of polarisations, so you can't just pick a direction of polarisation for it to have before it hits the polariser."