Birefringence is the effect that a material has different refractive indices for light of different orientations of polarization. A good way to visualize birefringence is by taking photos with cross polarized light, which can be achieved by sandwiching the sample between two polarizers, with the polarizers rotated 90 degrees with respect to each other. So schematically the setup will look something like this:


[light source] => [polarizer] => [sample] => [polarizer at 90 degrees] => [camera]


This means that any light from the light source travelling via the first polarizer will be blocked by the second polarizer since the polarizers are oriented perpendicalur to each other. However, if the sample material happens to be birefringent, then the polarized light will be split into two waves by the material, with those two directions being perpendicular to each other. One will vibrate along the plane of high refractive index, and one along the plane of low refractive index. While travelling through the material, these two waves will be in different phases since one wave is faster than the other. This difference in phase is called retardation, and it depends on the difference of refractive indices, the wavelength of the light and the thickness of the material. Upon exiting the crystal, the two waves recombine again, and depending on the amount of retardation, the direction of polarization will have changed by a certain amount by now. If the retardation is one wavelength, then the direction of polarization remains the same, but if the retardation is half a wavelength, then the direction of polarization will have rotated by 90 degrees.

The second polarizer will now split the light coming from the sample into two vectors, and only the vector parallel with the direction of polarization of the filter will be transmitted to the camera. And since different wavelengths will have different directions of polarization, some will have more light transmitted by the second polarizer then others, which is why colors show up in the sample.

If the retardation is little (either because the birefringence of the material is low, or because the sample is very thin) then the color will be almost white, turning sequentially into yellow, red, purple and blue with increasing retardation. An approximation of this order can be seen in the chart above, and it can even be seen in some of the photos below. Upon increasing retardation the colors will eventually turn into just shades of green and pink and, finally, upon even larger retardation, it will turn into "high order white".



Citric acid






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Sodium acetate

Sodium saccharin



Vitamin C