A Systematic Investigation of Optical Activity in Sugar Solutions

Mara Anderson, Dickenson College,
Marty Cohen and John Noe,
Department of Physics and Astronomy,
Stony Brook University

This project was inspired by a dramatic optics demonstration: vibrant
colors appear when linearly polarized (LP) white light passes through
corn syrup and is viewed through a second polarizer sheet. The colors
cycle through pale yellow, aqua, blue, purple and orange as the
polarizer is rotated. These effects are due to the chirality (left-
or right-handed structure) of sugar molecules and their resulting
ability to rotate the plane of polarization of LP light, an effect
called optical activity. (The rotation comes about because LP light
is equivalent to a superposition of left and right circularly
polarized light, and these two forms of light experience slightly
different indices of refraction in an optically-active medium.)

We decided to systematically investigate the optical activity of fructose,
a left-handed sugar. The overall goal was to explain and predict the color
effects seen in the demonstration. Several types of measurements were
carried out with a variety of polarized lasers: 633 nm red HeNe; 594 nm
yellow HeNe; 532 nm green DPSS; 488 nm blue Ar ion; and 404 nm violet
diode. The optical activity was determined from the angle through which a
rotatable polarizer needed to be turned to precisely offset the activity.
In one set of experiments, we first varied the fructose concentration in a
water solution with constant path length and then later varied the path
length while the concentration was kept constant. Other experiments
explored the dependence of rotation angle on wavelength in both fructose
and corn syrup, a mixture of fructose and the right-handed sugar dextrose
(glucose).

Our results for the path length and concentration experiments are as
expected: the optical activity varies linearly with either variable. To
obtain a linear relationship with respect to concentration it is important
that the concentration be recorded as grams of sugar per volume of final
solution. We also found that the rotation for constant concentration and
path length varies as 1/lambda^2. This unexpectedly strong wavelength
dependence is in agreement with results subsequently found in a literature
search.

This research was supported by a grant from the National Science Foundation
(PHY-0851594).