What is Birefringence?
Birefringence is an effect that causes psychedelic rainbow colours to appear in otherwise transparent materials. Such as the magnifying glass in the pictures above.
But actually, it’s a little more complicated than that:
Birefringent materials (mostly crystals) split an entering light ray into two rays that are both polarized with the vibration directions oriented at right angles to one another, and traveling at different velocities. When placed between two crossed polarizing filters, such materials appear in bright colors. With only polarized light entering the subject, one wave gets retarded with respect to the other and interference occurs between the waves as they pass through the second polarizer.
Even though you can create fantastic results just using two CPL filters, best results will be achieved with linear polarizers.