Software is presented for teaching elementary optics using a transparent semicircle. We demonstrate
the use of the semicircle to investigate Snell’s lawand students can are presented with the
difficulties involved in experiments. An Excel spreadsheet can show to students that small errors in
positioning of the semicircle can result in a non-constant index of refraction. Students can study
the effect of changing some of the parameters of placement of a semicircle on the accuracy of the
experimental results. They can see from the analysis of data that much better results are obtained
by doing regression analysis rather than by just taking the average value of the index of
refraction. Measuring the critical angle also gives a method of calculating the index of refraction.
Another way to measure the index of refraction is the use of the semicircle as a lens and from its
focal length we can deduce the index of refraction.