Cross polarisation is what happens when light travels between two polarisation filters. In this case, I use my computer monitor (a plain white screen) as a light source and a circular polariser on the camera lens with the object between the two.
It's the natural effect of the cross polarisation which creates the coloured patterns which reveal the stresses in clear plastic (no Photoshop) :
It's the natural effect of the cross polarisation which creates the coloured patterns which reveal the stresses in clear plastic (no Photoshop) :
