Optical Rotatory Dispersion in Quartz -- A Demonstration -- by Jon B. Applequist

The phenomena of optical rotation and spectral dispersion of optical rotation were discovered in 1812 by Jean Baptiste Biot, who observed them in quartz crystals. The experiments that showed colorful effects with polarized light were done by Biot's colleague Francois Arago in 1811, but Biot was the first to study them in detail and interpret them as being due to rotation of the light beam.

Experiments of the type performed by Biot are illustrated in the photographs above. Two clear quartz disks, 5 mm thick and cut perpendicular to the optic axis of the crystal, are placed between Polaroid sheets, which replace a reflecting glass and calcite analyzing prism in Biot's apparatus. The white background light is polarized vertically by the first sheet. The light emerging from the quartz disks appears colored when viewed through the second sheet, which is the dark strip across each photograph. The second sheet transmits only light polarized parallel to its long dimension. Thus, the sheet outside the quartz disks is light gray when oriented vertically and becomes black when it is horizontal.

Biot showed that the appearance of colors comes about because the direction of polarization of the transmitted light is rotated by the quartz about the direction of travel of the light, and each wavelength is rotated through a different angle. This variation is called optical rotatory dispersion. The color seen at a particular orientation of the second Polaroid sheet is that whose dominant wavelength is rotated to correspond to the direction of the sheet.

The two disks in this experiment were taken from two crystals which were identical in most respects, except that they were mirror images of each other. The two forms were discovered by Biot in the course of his study of quartz. They are known as left-handed quartz and right-handed quartz. The colors differ in the two forms because they rotate polarized light in opposite directions. In the above photographs it can be seen that the light which passes through both disks has no color, because the rotations of the two discs cancel each other at all wavelengths.

The phenomenon of optical rotation has been of enormous importance in physics, chemistry, and biology over the past two centuries, as it gave new insights into the physics of light, the structure of molecules, and the nature of living matter. The interested reader will find more on the history of this subject in my article "Optical Activity: Biot's Bequest", in American Scientist, 75:58 (1987). If you would like a reprint, just email me your mailing address. My email: jbaATiastate.edu (change AT to @).