A Systematic Investigation of Optical
Activity in Sugar Solutions
Mara Anderson, Dickenson College
Marty Cohen and John Noé,
Laser Teaching Center, Stony Brook University
Starting in my freshman year of high school, I had two job aspirations:
become a professional dancer and then one day retire and teach math.
Along the way to fulfilling those goals I took my first physics class and
decided that teaching physics was by far the more interesting choice,
seeing as how the student gets a hands-on real-world experience with the
Teaching optics in high school is always difficult. In
mechanics labs students get to see hands-on demonstrations where pieces
move and objects crash into each other. In electronics there are colorful
wires and devices that cause bulbs to light up, or sounds to go off.
There just are not as many bells and whistles involved in demonstrating
the properties of light waves. In this simple classroom experiment
involving linear polarizers and corn syrup, a bright and colorful display
catches attention and teaches a number of valuable lessons from how a
polarizer works to what makes the colors our eyes detect.
Elliptically polarized light occurs when the x and y components of the
electric field vector change over time. This can lead to two special
cases: if the components change over time but with the same phase,
linearly polarized light is created; if the components are 90degrees out
of phase, circularly polarized light is created.
Chirality is a property of molecules that can only
be easily observed when linearly polarized light is used. Chiral
molecules can have a handedness to their structure; one can be a mirror
image of another but with the carbon bonds arranged differently. This
difference in structure will cause light passing through the molecule to
Due to the difference in refraction index, light entering a
solution will be slowed down. The fascinating aspect of chirality (for
our color construction) is that the refraction index will be greater for
right circularly polarized light than for left (if the molecule is left
handed), causing a phase difference between the two components and thus
rotating the linearly polarized light.
The angle the rotated light makes with the first polarizer is dependent on
a number of factors: path length, concentration, temperature and
where alpha is the specific rotation, T is the room temperature in Celsius,
lambda is the wavelength, l is the path length in dm, and d is the
concentration in g/mL.
We worked with fructose in order to avoid the mixed
of the combination of optically active molecules in corn syrup. In order to
figure out the effect of concentration and path length, two setups were
created and subsequently tested with a range of
| Laser Type
|Blue Argon Ion
In the first, path length was held constant while concentration was
(A laser beam is passed through
a polarizer and then recieved by a photon detector.)
This was followed by measurements at a constant concentration but variable
The data produced from these experiments will appear curved
linear, so it as also important to account for the change in volume of the
sugar-water solution. There is no listed source for calculating the
increase, so we put together a simple experiment in a graduated cylinder
and found a linear relation, namely:
, where y is volume of solution and x is grams of sugar.
Combining all of
these linear concentration-path-length dependent
functions, we produce a graph where we can easily see the dependence on
After several attempts, this dependence was determined to be
The dependence we determined also happens to lie very
close to later found literature values.
Similar measurements were taken with each wavelength passing through the
corn syrup (at a constant path length). Although it does not give us the
path length and concentration dependence, using the slope of this
relation, we can finally explain the colors we see in the original
demonstration. We use the Law of Malus to determine the intensity of the
different wavelengths at different angles of the second polarizer.
Where I is the final intensity of the wavelength,
I0 is the initial intensity, θα is the
specific rotation and
the angle of the second polarizer to the first.
concentration (roughly 73g of fructose in the original 50mL of
| Laser Type
|Blue Argon Ion
Recreating the Demonstration
Spreadsheet model etc ...
Calculated spectra (intensity vs. wavelength plots) for settings of the
second polarizer of 0, 30, 45, 60, 90, 120, 135, 150 and 180 degrees
relative to the
first polarizer. The sequence runs from left to right, top to bottom.