Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] unbiased experiments +- index of refraction



The only problem I have with totally eschewing "accepted-value" labs is
that students should learn to calibrate their instruments. Thus, the
accepted-value lab is the starting place to make sure the equipment is
working properly. The accepted-value lab should not end once the
student has determined how well the accepted value was measured.

One of the things I like about the "refractive index lab/demonstration"
I described is that when measuring sugar content in drinks we don't
actually measure a numerical refractive index and compare it to any
published refractive index values. Rather, students calibrate the
homemade refractometer with sugar solutions they create, and then use
the calibrated refractometer to measure some solutions for which they
don't know the sugar content.

If you have time, you can make the lab even more productive by adding
several components. One component is to have the students calibrate the
refractometer with solutions they make, then have them measure some
"unknowns" you made. This simulates a common practice performed in many
commercial labs that periodically have to be recertified. The
commercial lab calibrates their instruments using their usual methods,
then they have to determine several "unknowns" provided by the state or
federal EPA or other certifying agency.

Or, an in-house lab in an industry such as a refinery might test their
waste-water effluent for various things, and they periodically have to
split the samples and analyze them both in-house and by hiring an
outside EPA-certified laboratory, and the numbers need to agree within
some tolerance.

Another component that can be added to the refraction-measured
sugar-water is to find out how much the results differ if the
calibration and unknowns are not measured at the same temperature. Can
you calibrate at room temperature and then measure a can of Coke
straight from the fridge? (No.) Can you calibrate with
refrigerator-temperature standards and then measure a can of Coke
straight from the fridge? (Yes.)

For more advanced classes, students can use a hollow prism on a
spectrometer and find out what things they need to control in order to
get a refractive index measurement that agrees with a published value
within three decimal places. It's not difficult to get this level of
accuracy, but it's not a piece of cake either. For example, if the
students are not told that published values are measured with 589-nm
light, then using a He-Ne laser they might get good agreement with a
low-dispersion liquid such as water, but poor agreement with a
high-dispersion liquid such as toluene. Can they figure this out on
their own?


Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu