Aspirin crystals evaporated from ethol alcohol solution. Copyright Thomas L. Webster 2004

The physics made very simple... Light rays are composed of waves of light that vibrate in all planes. When light rays pass through a polarizing filter only those light rays that are vibrating parallel to the privileged direction of the polarizing filter are able to pass through the filter. This results in light rays that are all vibrating in the same plane of travel. The more oblique the light rays that strike the front surface of the filter the more these oblique light rays are filtered out. Perhaps you have noticed in your own photography that polarizing filters have their greatest effect on a landscape photograph when the scene being photographed is oriented 90° to the sun. In this orientation, the polarizing filter is blocking out the majority of oblique light rays that contribute to reflections and flare and thereby increases the saturation of the photograph.

Polarizing filters come in rotating mounts that allow the photographer to orient the privileged direction of the polarizing filter to obtain the highest degree of polarization. If the photographer were to "stack" two polarizing filters together, view a strong light source through the stacked filters, and then rotate the filters, the photographer would see that the light source would become dimmer and dimmer as the privileged direction of the two polarizing filters approach 90° in relationship to each other. Theoretically, when the privileged direction of the two filters cross at 90°, all light passing through the filters will blocked (extinguished) and the photographer would no longer be able to see the light source through the filters. In this orientation, the polarizing filters are said to be crossed, hence the term crossed-polarization. In reality, however, due to the quantum nature of light, a faintly visible and blue-shifted light source can still be seen. For photographic purposes, though, the extinction of the light is complete enough.

The splitting of light into two components (an ordinary light ray and an extraordinary light ray) by a crystaline substance is known as birefringence. Birefringence is also known as "double refraction". Birefringence is the result of the crystal material having two indices of refraction. One light ray is slowed down and color shifted compared to another light ray. Birefringence is caused by the atoms in a crystal having stronger bonds with one another in one direction and weaker bonds with one another in a second direction.

If all of the transmitted light is being extinguished by the crossed-polarizing filters, then how can a photographer photograph crystals under the microscope? As regards polarized light, birefringent crystals come in two "flavors". When struck by strongly polarized light, isotropic crystals allow transmitted light to pass through the crystal relatively unaltered. Since the second polarizing filter is crossed 90° in regards to the first polarizing filter, a photographer would not be able to view these crystals with crossed-polarizing filters. Anisotropic crystals, on the other hand, act as tiny prisms that break up transmitted white light into its constituent wavelengths of red, blue, green, yellow, and violet light. Uniquely, anisotropic crystals not only break up white light into its constituent wavelengths but anisotropic crystals rotate the constituent wavelengths 90° to plane of the polarized light transmitted through the crystals. The constituent light rays are then able to pass through the second polarizing filter and expose the film in the camera. The results are truly kaleidoscopic! (Continue to Part II...)