A trapezoidal tank, lit from behind by polarized light from a rotating polarizer, contains various examples of optically active materials, including stressed glass. As the rear polaroid rotates a fixed polaroid on the front of the tank makes the patterns visible. Click your mouse on the photograph above to see a short mpeg video clip of the setup in motion.

This is a wonderful hallway demonstration.

The prism spectrum part of the experiment consists of a bright point source with condenser lens and iris, a 20 cm focal length cylindrical convex lens which focuses light onto a slit, and a 20 cm focal length spherical lens which focuses the light passing through a prism onto a distant screen. After the spectrum is aligned, the optical activity arrangement, consisting of a uniform plastic cylinder of Karo syrup between two rotatable polaroids, can be positioned between the cylindrical lens and the slit, as seen in the photograph. As either polaroid is rotated a dark band moves through the spectrum, because the polaroid absorbs the color/wavelength that has rotated by that angle.

The color of the light passing through the polaroid/Karo syrup system is a series of negative colors: white minus the color removed by the polaroids. The spectra are shown above (left to right): white light, cyan (minus red), magenta (minus green), and yellow (minus blue) respectively.

Light from a bright point source with condenser and iris passes through a yellow filter and two crossed polaroids onto a distant screen. The quarter-wave plate is then positioned between the two crossed polaroids and rotated to any angle except 45 degrees with respect to the crossed polaroids. When the analyzer is rotated periodic minima and maxima of illumination appear on the screen, because the beam is elliptically polarized, and no position of the analyzer will produce darkness. When the polarizer and analyzer are crossed and the quarter-wave plate is set at 45 degrees, rotating the analyzer does not produce any changes in the intensity of the beam. This is the effect of circular polarization.

Note that the yellow filter is used because the thickness of the quarter wave plate is based on the wavelength of yellow light.

Light from a bright point source with condenser and iris passes through a yellow filter and two crossed polaroids onto a distant screen. The half-wave plate, consisting of two identical quarter-wave plates, is then positioned between the two crossed polaroids at the angle which produces the greatest illumination on the screen. Remove the plate and cross the polaroids. The half-wave plate is then inserted set at the angle for darkness. The plate is rotated somewhat less than 45 degrees, producing some light. The analyzer must be rotated by twice the angle of the plate rotation to again produce darkness.

The quarter wave plates have thickness chosen for yellow light.