2 days. Hands-on activity using the actual path of light through your container of water. Students compute the speed of light in water and develop an understanding of why light reflects back if below a critical angle. It provides an opportunity to build a fairly complicated function from simpler ones in a physical setting, and then investigate the function to learn something about the real world. It involves investigation of function with a parameter. This unit is appropriate for a precalculus course in a unit on functions or or in a unit on functions involving radicals. It has also been used in elementary calculus courses as a conceptual introduction to optimization.