You and a friend are scuba diving on a calm day when you decide it would be neat to estimate the critical angle for light going from water into air. Your friend is underwater and has a laser pointer and a waterproof earpiece so that he can hear your instructions. He also has a floating marker attached to him that remains on the surface directly above him. Sitting in a small motorboat on the surface, you have a variety of tools for measuring distances (tape measures, fishing line that could be let out and later accurately measured, and so on). You think for a moment and decide on a course of action.