Illustration of Plane-Polarized Light Introduction of the topic of optical activity to sophomore organic chemistry students invariably includes a discussion of the workings of a polarimeter. Most textbooks include an illustration which shows the polarimeter source light polarized by one lens and the extent of rotation of the plane of this plane-polarized light detected by a second lens. For small classes the principle of the polarimeter is easily illustrated by holding up before the class two 6- X 6-in. polarizing lenses (available from Edmond Scientific and several other sources) and then rotating one lens with respect to the other. With large classes, however, not all students can see through the lenses from where they are seated. An overhead projector solves this problem. Two polarizing lenses, with their polarizing planes oriented in the same direction are placed on the overhead projector viewing surface which has class notes already written on it (a). As one lens is rotated, theview of the notes is obscured (b), until, after rotation of go", one lens exactly blocks out the light allowed ta pass through the other lens (c). Even in a large classroom, this effect is easily seen by all in a very dramatic way, and the concept of plane-polarized light is clearly exemplified. In the polarimeter, the observed rotation angle is that angle that the second lens must be rotated to allow maximum passage of light.
This very simple illustration of the effect of polarizing lenses could be expanded to a lecture demonstration such as that described by Slahaugh' to demonstrate the dependence of the observed rotation of an optically active solution on the concentration of that solution.
' Slabaugh, W. H. J. Chem. Educ, 1967,44. A146. Stella D. Elakovlch The University of Southern Mississippi Haniesburg. MS 39406
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