Harvard Natural Sciences Lecture Demonstrations conducted a demonstration of an optical technique that allows us to see small changes in the index of refraction in air, which made visible by Schlieren Optics. 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.
Watch the heated gases from a candle flame and a hair dryer, helium gas, and sulfur hexafluoride gas in the demonstration video below.