overhead projector
edited by DORIS KOLB Bradley University Peoria. IL 61625
Optical Activity: An Improved Demonstration
A Simple Tyndall Effect Experiment
Gordon F. Hambly John Abbott College, Ste Anne de Bellevue. W H9X 3L9. Canada
Recently, Kolb' mentioned an overhead projector demonstration of optical activity described earlier by N01ler.~A variation has also been reported by Hi11.3 The following modifications render the demonstration more imnressive and instructive. The olastic base of a 100-mL eraduated cvlinder is removed,and a few plastic rings areadded to block out extraneous light. In the center of a piece of cardboard large enough to cover the overhead projector surface, a hole is cut slightly smaller in diameter than the graduated cylinder. A Polaroid sheet is cut into two equal rectangles; after a 90° rotation of one, no light will pass through; cardboard pointers are then attached to corresponding edges. When the graduated cylinder is filled with water and the cardboard pointers on the Polaroid sheets above and below the graduated cylinder are parallel, no light is transmitted (zero rotation). A solution bf 50 g of sucrose in 50 mL of water, recently prepared, is then placed in the graduated cylinder. In a sufficiently darkened room, with the extraneous projector light eliminated as described above, a beautiful ohenomenon is observed as the top Polaroid sheet is slowiy rotated: the image on the screen never totally disappears, but it goes through the colors of the rainbow. Students can be asked about what factors affect the observed rotation of a solution of a chiral compound. The path length and the concentration are normally the first two responses. When questioned about the "rainbow phenomenon", they recognize, often with a little prodding, that different wavelengths of light from the overhead bulb are rotated by different amounts. This leads naturally into a discussion of standardizing optical rotations and the need to specify the light source (D line of sodium, 589 nm).
Robert H. Goldsrnlth St. Mary's College of Maryland St. Mary's City. MD 20666
The overhead oroiector can be used in a simnle demonstration to illustiate the Tyndall effect and the difference between transmitted and reflected lieht. The elass surface of the projector is covered with a pieceof cardgoard or heavy paper with a hole cut in the center about the size of a 400- or 600-mL beaker. The beaker is placed directly over the opening and filled one-half to two-thirds full of 10% sodium thiosulfate (Na2S203)solution. The machine is focused so that a clear circular spot of light is projected onto the screen. The instructor then adds 1M HC1 solution very slowly, one dropat a time. Students are soon able to see the light reflected by the colloidal particles of sulfur as they begin to form in the beaker. As more acid is slowly added, the amount of reflected liaht (as seen in the beaker) increases, while the amount of transmitted light (as observed on the projected circle of light) decreases. Eventually all of the light is reflected by thesulfur particles, and none is ohserved on the screen, indicating that no liaht is now heine transmitted. Carrvine out this demonstratron on the overgead projector makks more dramatic and more visible. I t also illustrates more clearly the difference between reflected and transmitted light than do other approaches, such as shining a flashlight through a colloidal suspension.
' Kolb, D. J. J. Chem. Educ. 1987, 64, 805.
Noller, C. R. J. Chem. Educ. 1949, 26, 269. Hill, J. W. J. Chem. Educ. 1973, 50, 574.
Volume 65
Number 7
July 1988
623
