It is easy to get beautiful multicolored images using a set of polarizing filters. These filters act on the electromagnetic nature of light by passing only that energy whose electric field lies along a particular direction, known as the polarization axis of the filter. A double filter made by rotating two polarizers so that their axes are perpendicular should block all light. Odd as this may sound, adding a third polarizer between them can cause light to be transmitted again. Some light will pass through the added filter because its polarization axis is not at right angles to the axis of the first filter. Whatever light does get through must be aligned with the intermediate filter's axis. In effect, this filter rotates the electric field. Therefore, when the rotated light reaches the final filter, some can pass through.
Most crystals also polarize light because of the highly regular arrangement of their atoms. A crystal generally polarizes different wavelengths with different efficiencies, which causes some colors to come through more vividly than others. To capture this effect on film, position one polarizing filter above the light source and attach another to the objective lens of your microscope. Most large scientific supply houses carry such filters. Fisher Science Education sells a kit for $23 that allows you to construct both filters and even contains some crystalline material to study (item number CQS19709-3). Or you can experiment with making your own filters. Edmund Scientific in Barrington, N.J., has available high-quality polarizing, thin plastic sheets for $6 (call 609-573-6250; catalogue number H43781).
To study crystallization, place on the stage a material with a melting point between room temperature and 100 degrees C. By tuning the power supply to keep the stage temperature near the melting point of that material, you can alternately create and destroy the crystals by making small adjustments to the power. This experiment is much more satisfying than, say, watching salt crystals form by evaporation, during which you get to see such crystals only once before you must clean and reset the stage.
There are a number of materials that are safe to melt on the stage. Silk recommended thymol, camphor, menthol, stearic acid, trimyristin or myristic acid. You can obtain a mixture of trimyristin and myristic acid by soaking nutmeg in methanol and then filtering and evaporating the solvent. Most of the other substances you should be able to find in drugstores and chemistry sets. Consult the Merck Index for other ideas.
Of course, be careful. Some materials, such as naphthalene (once the active ingredient in mothballs), emit harmful gases when heated. (Modern mothballs often use paradichlorobenzene, which, though classified as an irritant, is not considered toxic to humans. And paradichlorobenzene crystals are a delight to study.) Never experiment with something unless you know it is safe or you are sure that nobody could be exposed to potentially poisonous fumes. Dedicated experimenters may want to invent a way to seal the cell hermetically or vent the vapors. A sealed cell could be used to incubate thermophilic organisms. I invite you to share your ideas on the relevant discussion area at the Society for Amateur Scientists Web page.
As a service to the amateur community, the Society for Amateur Scientists can provide the conductive glass for this project for $12 plus $3 shipping until May 2000. To place an order, call 619-239-8807 or send a check to 4735 Clairemont Square, Suite 179, San Diego, CA 92117. For more information about this and other projects from this department, check out the Society for Amateur Scientists Web page at www.thesphere.com/SAS/WebX.cgi.
Because of a printing error, labels were omitted from an illustration in April's Amateur Scientist. The full diagram is available at http://www.sciam.com/article.cfm?articleID=0003A0E1-5575-1C70-84A9809EC588EF21 on the World Wide Web. Scientific American apologizes for the inconvenience.
This article was originally published with the title Hot Views of the Microscopic World.
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