Thank you for sharing this excellent work you have done. This is a tremendous resource for understanding physics! I am not a physicist, but I am working on a solar energy project and need to understand something about optics for concentrating sunlight with a lens, including the prismatic effect, as demonstrated by your rainbow applet. I hope to be able to create an applet of the project I'm working on, building on the 'thick lens' and the 'rainbow'. If I am successful, I'll post it for others to see.