For the lazy:
I did a project in high school physics on the phenomenon of birefringence in combination with polarization. Take a clear sheet and a light source (we used transparencies and an overhead projector). Put loads of scotch tape haphazardly on the clear sheet (or, you know, the inside of your visor). When I say haphazardly, make sure it's good and flat but otherwise criss-cross it, make shapes, etc. If it's all going in one direction up and down, it's going to be boring. Put a polarizing filter between the sheet and the light source (or hold your scotch-taped visor up to the sun); we put thetransparency on top of a large polarizing filter sitting on the surface of the overhead projector. Then put another polarizing filter (your sunglasses, for example) between the transparency and the observer (your eyes or a projection screen) and rotate that polarizing filter. Congratulations, you've just created a Polarimandalascope.
Oh, and the trick is this: The tape is birefringent, thanks to the process of stretching by which it's made. In different layers, this alters the amount the light (which is already polarized) is bent. What you actually end up with is circular polarization, so that when you rotate the second polarizing filter, different frequencies (i.e., colors) of light have rotated different amounts and can therefore pass through. So what you see is a wide array of colors in all sorts of patterns, based on how the tape was applied.