Schlieren Optics shows how air moves, the results are pretty damn cool

Demonstration of an optical technique that allows us to see small changes in the index of refraction in air. A point source of light is reflected from a concave mirror and focused onto the edge of a razor blade, which is mounted in front of the camera. Light refracted near the mirror and intercepted by the blade gives the illusion of a shadow. Seen here are the heated gases from a candle flame and a hair dryer, helium gas, and sulfur hexafluoride gas.