If you've heard of Fresnel lenses in photography before but have no idea what they are or how they work, here's a fantastic short primer. In this 3.5-minute video, physicist David G. Willey (AKA the Mad Scientist) explains the science behind the lens style using a simple and easy-to-understand model on a blackboard.
Schlieren imaging is a fairly standard optical technique for visualizing heat, sound, or pressure differences in air. Technically, a schlieren system is able to see clear disturbances in air due to slight differences in the speed of light in the air. The technique can be sensitive enough to see the heat rising off a human hand at room temperature.
Studio lighting can get hot -- so hot, in fact, that some can double a stove. Here's a humorous 14-minute video in which photographer Sweet Lou tests how well different ARRI fresnel lights fry eggs and bake cookies.