The Art of Crystalography

Crystals in Polarized Light

I. The basics

For simplification, light can be considered as a form of electromagnetic energy vibrating in all directions, or in other words, in all planes. Polarized light vibrates only along one plane, as along the surface of a piece of paper. The polarization of light occurs naturally, a good example is the rainbow, where the colors of light which are separated by the prism effect of rain droplets, vibrate in one direction. Polarization of light can occur by passing through various media, or by reflection off various surfaces or by refraction. Polarization of starlight also occurs in outer space as light passes through dust and gas clouds. In the laboratory, light is most commonly polarized by passing it through polarizing filters, the molecules of which are long-chain polymers arranged parallel to one-another. Therefore only light which vibrates in the same direction the filter molecules are arranged will pass through the filter...that is, the light has to be vibrating parallel to the crystal lattice of the filter. All other light is blocked. This reduces light scatter (vibration in all directions) and provides less visual glare for the human eye. Polarizing sunglasses function in this capacity, each lens effectively is a polarizing filter. If two polarizing filters are held in front of the eye one behind the other with a light source behind them and one of the two lenses is rotated, at one point no light will pass through the two lenses and they will turn dark. This is because while rotating the lenses, the observer will arrange the lenses such that the elongated molecules of one lens are at 90 degree angles to those of the other, thus blocking out the light.

II. So what about crystals?

Crystalline substances, not all, have a property called birefingence which causes them to rotate light as it passes through their crystal matrices. Therefore if a crystal of a birefringent substance is placed between two polarizing filters which have been arranged as above to extinquish light transmission, the observer will see the crystal as it rotates plane polarized light enough to allow it to pass through the second filter (closest to the eye). Often the crystal will rotate only certain colors so that the observer will see a pleasing display of colors.

III. How the images are made.

A professional quality binocular microscope is the basic instrument. It is equipped with distortion-free polarizing filters to create a black field of view through the eyepieces, and flat field optics to provide sharp focus to the edges of the image. The scope has a trinocular head (three optical pathways)...2 pathways for the eyepieces and the third for an attached 32 megapixel digital camera which is connected via USB-2 to a computer. Crystals are grown on a microscope slide and then viewed through the microscope. Selected scenes are then captured and stored on the computer and backed up on an external drive.A wide variety of color backgrounds can be achieved by manipulating the filters. Prints are currently made with an EPSON 5500 large format ink jet printer which will print up to 14 x 20 inches, the dyes will remain color-fast up to 100 years. I am currently looking into a second printer with up-to-date print heads and new brilliant dyes for prints up to 30 x 40 inches.

The substances used for photography all have a melting point low enough to be reached by an alcohol lamp. Other substances will not melt or will either decompose or melt at temperatures dangerously high. Melting is preferable, however some difficult to melt substances might dissolve in water and crystals can be grown by evaporation of a solution. The substances represent familiar household chemicals such as sodium bicarbonate (baking soda) and other crystalline substances used in hospital laboratories and in various industrial laboratories. The substances are melted on a glass slide and upon cooling, they crystallize, at which point they are examined with a polarizing photo-microscope. Crystals grown by evaporation are similarly evaluated. Suitable areas are chosen and the images are captured with a 32 Megapixel camera attached to a computer. Images are about 90 MB and stored as TIFF files. Older images were taken via a slide film system (Kodachrome and Ektachrome) and subsequently scanned with a Nikon 4000 transparency scanner at 22 MP TIFF, this will ordinarily limit prints to not much more than 14 X 20 inches, although larger prints viewed at 3 or more feet may look sharp enough.