A TV Lecture Demonstration
Oiler 1. Henderson
Eastern Illinois University Charleston, Illinois 61920
of Optical Activity
The topic of optical activity frequently arises in the undergraduate class. I n many instances it is cumbersome or even impossible to demonstrate the phenomenon to the contemporary large chemistry class by conventional means. A satisfactory demonstration has been arranged by utilizing a closed-circuit television in conjunction with polarimeter optics. 100 Watt Lamp
TV Camera
0
8
"
L, .Condenser lens
Figure 2. Optical ry5tem. From left to right: vorioble intensity light; condenser; polarizer; romple cell; analyzer; TV camera.
Monitw
L2.0ptional objective lens PI ,P, .Polarizing filters T .Variable voltage transformer F= Focal length of L, Figure 1.
provided with a variable transformer connected to the incandescent lamp. Results
Optical system.
Optical System
The heterogeneous light from an incandescent bulb is collimated by the lens L (See Figs. 1 and 2). The condenser may be any convenient lens of appropriate diameter and focal length.' Polarizing filters are readily available from most optic suppliers and do not need to be of high quality. (Polarizers mounted in a rotatable fmme and calibrated in degrees are available from LaPme Scientific Co., Chicago.) The cell may be ~onst~ructed from a piece of stock 20-mm tubing, provided with a side arm for introducing liquid samples. The ends of the cell should be ground parallel and equipped with flat glass windows which may he cemented in place with an epoxy resim2 Sodium chlorate cryst d s are an excellent choice t,o illustrate optical activity in the solid state sinre they possess a ouhir habit and A may be easily grown by conventional means.3 The t e l e ~ i m n camera should be equipped with a close-up lens, focused on the arkilywr so as to produce a magnified picI ture of the light field and Figure 5. Leva- ond dertrorotaintervening sample, tory NaC101 crystals crossed poltensity control may be arize=,
Optical activity has been very satisfactorily demonstrated in aqueous samples of carbohydrates (See Figs. 3 and 4). Optical activity arising from lattice dissymmetry may be conveniently illustrated by replacing the sample cell with dextro- and levorotatory
Figure 3. A romple of glvcore 301"lion between crossed polarizerl lo, disployed by TV monitor).
Figure 4. Glucore sample with anolyzer rototed +14'.
I
-
Figure 6. NoC103 crystals with (Note anolyler rotated -30'. extinction in levorotmory uyst0i.l
Figure 7. NoC10x uyltalr with onolyrer rotated +30°.
Volume 44, Number 12, December 1967
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crystals of NaC10~(See Figs. 5, 6, and 7). The origin of dissymmetry can be revealed by demonstrating the lack of activity in NaCIOs solutions. By employing crystals of different thickness it is possible to show the dependence of rotatory power on optical path length. A white-light source will of course give rise to rotatory dispersion colors at the analyzer. However, this is of no serious consequence for a qualitative demonstration since the camera displays only the contrast in light intensity between the field and the sample. Since a rather large number of the new chemistry
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lournal of Chemical Education
lecture facilities have incorporated closed circuit television, this technique shouldprovide an excellent means of demonstrating optical activity to the large classes. 'The diameter should be approximately equivalent to, or larger than, the aperture of the polarieer, p,. A focal length of 1030 cm would afford a convenient working distance. 'Inexpensive flat glass windows of various diameters are available from Edmond Scientific Co., Barrington, N. J. HOLDEN, ALIN, 4ND SINGER,PHYLIS, "Crystals and Cry8td Growing," Anchor Books Doubleday and Co. Inc., Garden City, New Yark 1960, p. 115.
