|Screen shot is from the Phet site at the linked refraction demo.|
Before I start talking about an awesome simulation, hear the standard disclaimer: Online simulations are in no way a substitute for live quantitative demonstrations.
That said, online simulations, if they're programmed correctly, can be extraordinarily useful: for making quick "measurements," for showing experiments and regimes within an experiment for which you don't have the equipment, for student use at home... As long as you are not trying to replace live equipment with a computer, simulations are wonderful resources.
The Phet interactive simulation site is one of the traditional favorites of physics teachers. These have been maintained and developed over time by pros. Note that they are free (with donations accepted), and that they require Java.
Today my conceptual class used a laser and a fish tank to make measurements of incident, reflected, and refracted angles. Homework questions will ask qualitatively about which way light bends at various interfaces, about comparing angles, about how these angles change in different situations.
My colleague Alex Tisch whipped out this phet simulation which runs exactly the same way as my in-class live demonstration. It even comes with a protractor that you have to place properly to measure angles. I particularly love the option of a "mystery material" for which you have to use the protractor to figure out the index of refraction.*
* In Regents or AP physics, I'd have students use Snell's law to determine the mystery index of refraction. For conceptual, I could ask students to rank materials by their n, or to compare the material's index of refraction to that of water, say.
I'm not actually using this for any sort of official assignment, at least for now. Rather, I just put a link on the class folder, and offered extra credit to anyone who actually downloads and plays with the simulation tonight. If nothing else, I might use it myself in creating a problem -- a screen shot provides me a diagram from which I can ask virtually anything. Alex did the live demonstration, then used the simulation to make many quick measurements without having to turn out the lights, click erasers to visualize the laser, draw the rays on the glass, etc.
Thanks for the link, Alex!