Polaroids in a row

You will write three small functions for a row of Polaroid sheets.

The idea. Unpolarised light loses half of its intensity in the first Polaroid. After that the light is polarised along the pass direction of the sheet it just left. At the next sheet, Malus' law gives $I = I_p\cos^2\theta$, with $\theta$ the angle between the two pass directions.

Your tasks.

1. In after_first, return the intensity after the first Polaroid when the light that falls on it is unpolarised.
2. In malus, return the intensity after an analyser at angle_deg degrees to the vibration of the polarised light of intensity intensity.
3. In chain, return the intensity after a row of Polaroids. angles is the list of the pass directions, in degrees. The first sheet takes unpolarised light.