Polariser and Analyser

When unpolarised light passes through a polaroid $P_1$, its vibrations are restricted to a single plane, emerging as plane polarised light; passing this beam through a second polaroid $P_2$ and rotating $P_2$ about the beam's own axis shows the transmitted intensity varying smoothly from a maximum (at one particular orientation) down to complete extinction after a $90°$ rotation, then back up to maximum after a further $90°$. The first polaroid $P_1$, which produces the plane polarised light, is called the polariser; the second polaroid $P_2$, used here to test whether a beam of light is polarised, is called the analyser. If the intensity of the original unpolarised light is $I$, the intensity of the plan …