Visualizing Air Flow Using Schlieren Optics
Harvard’s Department of Natural Sciences demonstrates an optical technique known as Schlieren Optics, 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. For more information on this specific setup, see here.
Note that this version of the setup uses a white LED flashlight instead of an automotive light bulb.