Cross-Polarisation is something I learnt in my Physics lessons.
It basically involves filtering the planes of direction at which the light travel.
The shot was taken using my computer screen with a white fill on. I then placed the set square up and then my camera with a polariser on.
As you twist the filter, the white screen slowly goes back, as the white light from the monitor is already polarised the light is only in one plane, vertical or horizontal. As you twist the filter, you gradually block off the vertical or horizontal plane so there is no light hitting the sensor. However, the plastic, distorts the light flow creating these great colours!
Its a technique that used to be used by engineers to test the structural integrity of buildings such as bridges, as you twist the plastic, the light behaves differently as it passes through as so changes the colour.