In nature, tourmaline exhibits the effect of polarizing light.

None of the materials grown in this experiment directly polarize light. However, sodium chlorate does have the effect that when placed between cross-polarizers will transmit different colors depending on the relative angle between the polarizers. This effect seems to be due to the fact that sodium chlorate crystals polarize different wavelengths of light at different angles relative the crystal face (but only when they enter the crystal polarized in the first place). The effect is reminiscent of a wave retarding plate, only gradient in nature depending on the wavelength of entering light. Picture shown below. The turns are through 180 degrees.

However, a puzzle remains. How does cubic sodium chlorate act like a wave retarder, when it possesses no birefringence by the nature of being cubic?

To learn more about Polarization (both linear and circular), check out a recent lab that Lee Burnett and I completed.