Measuring the refractive index of liquids generally comes up each year
about this time. I strongly recommend making your own hollow prisms and
using them with a laser pointer or with a spectrometer. Below I list
two papers I have published about this. In addition, I have a link to
directions for an easy way to make hollow prisms. This method of making
them works so well, and using the prisms works so well, I would say that
if you are serious about doing refractive index demonstrations or labs,
and you haven't tried this, you really need to try it. Once you've done
it, I'm sure you won't go back to anything else. I've used these all
the way from first graders to college students.
Here are some more hints:
(1) Make one or more hollow glass prisms.
(2) Learn the "minimum angle of refraction technique" for using the
prism.
(3) Use the prism with all kinds of liquids, but especially use water
and sugar water of various concentrations.
(4) After calibration with known sugar concentrations, use the prisms to
measure the sugar content of all kinds of drinks: Coke, Pepsi, Mountain
Dew, white grape juice, honey, you name it. The amount of sugar in Coke
and Pepsi will amaze the students. Also do Diet Coke/Pepsi.
(5) Read the two publications below. The more recent Journal of
Chemical Education publication is focused more on demonstrating the
principles and measuring sugar content of sugar-water solutions. The
older Physics Teacher article is more focused on coming up with an
actual number for the refractive index.
Here are some specific responses to other current posts on this
subject....
John Clement said: "Soda might be difficult because the bubbles will
disrupt the light. You could have the students make saturated solutions
of salt or sugar. I suspect the index will also be very close to that
of water."
Response: No, soda is very easy to measure, and the bubbles are easily
managed. If a bubble attaches to the glass at the point where the laser
beam is going through, simply dislodge the bubble with a spatula, wood
splint, toothpick, etc. The refractive index of sugar water is not
similar to water, and it is very easy to calibrate a system and then
measure the sugar concentration in various juices and soft drinks. The
minimum angle of deviation is a necessary component of this technique.
Please read the Journal of Chemical Education paper referenced below.
This is an outstanding lab experiment and/or demonstration.
Paul Lulai said: "For each liquid, they could measure multiple angles,
plot sin(theta-i) vs sin(theta-r), From the plot they could determine if
y-intercept or slopes have any meaning. This would have a better
influence if different groups measured these angles for different
materials (or each group did this for multiple materials). <snip> That
would be my h.s. approach. Curious if there is a more high-falutin
college or industrial approach."
Response: Paul's suggested method is very much the extremely hard way to
do this. My "college approach" is not "high falutin" but is incredibly
simple. I have used the demonstration of sugar in Coke with 1st
graders. In addition, high-school students and gen-ed college can
determine an accurate numerical refractive index. You only need to know
the angle of the prism (generally 60 degrees) and the
minimum-angle-of-deviation between the laser beam with the prism not
present, then with the prism present and adjusted to the minimum
deviation (straightest path). You really only measure one angle.
Please read the publications.
Curtis Osterhoudt said, "The index of concentrated sugar water or salt
water isn't *much* higher than for pure water..."
Response: I don't know where this is coming from. The demonstration of
the difference, when done properly, is dramatic. This is the way
vintners measure the sugar content of their grapes. You can buy
portable pocket refractometers for measuring sugar in water (also for
measuring salt in water). Unfortunately they are expensive. The J Chem
Ed publication explains how to make your own sugar-water
refractometer... but it won't fit in your pocket.
REFERENCES:
"A Liquid Prism for Refractive Index Studies", Edmiston, Michael D., J.
Chem. Educ., 78, p. 1479 (2001).
"Measuring Refractive Indices, Michael D. Edmiston, The Physics Teacher,
Volume 24, Issue 3, pp. 160-163 (1986).
Link for directions for hollow glass prism fabrication...
www.bluffton.edu/~edmistonm/hollow.prism.directions.pdf
Link of preprint of the J Chem Ed paper. This may be a working version
and may differ slightly from the published version (I did not check it
word-for-word).:
www.bluffton.edu/~edmistonm/liquid.prism.pdf
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu