1999 GSA Annual Meeting -- Denver, Colorado
1999 GSA Annual Meeting -- Denver, Colorado
Author(s): JOHNSON, Neil, E. Department of Geology, Appalachian State University, Boone, NC 28608-2067, johnsonne@appstate.edu
Keywords: Crystallography, X-Ray Diffraction
The conceptual leap to diffraction patterns from even simple point arrays has long been recognized as challenging. But for more than half a century it has also been recognized that an analogy between optical and X-Ray diffraction represents an ideal bridge, requiring only a source of monochromatic light and a finely-spaced array of scatterers. Inexpensive lasers have fulfilled the first requirement, but the second has been problematic, due to the difficulty in creating arrays with spacings of 50 - 100 micrometers. A number of approaches have been used, including pantograph reduction, several types of photographic reduction, and even standard sieves, but all require significant preparation, limiting student experimentation. Imaging technology has improved so that printers with resolutions of 600 or 1200 dots per inch (one dot per 42 or 21 micrometers) are inexpensive and available. Concomitant with this have been increases in the screen magnification capabilities of graphics software. The result is that students can create and modify their own patterns, print them on transparencies, and illuminate them with laser pointers. The turnaround time from idea to observation can be as little as a few minutes. Students are provided with samples that demonstrate the basic effects of changing spot distances, centering and the like. Subsequent examples can illustrate plane group symmetry, structural modulations or even powder diffraction. Furthermore, the obvious question What happens if I change this? can be answered Try it and see! Individuals or groups can be required to make class presentations on their discoveries; the possibilities are truly endless.
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