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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.  Schlieren (German, meaning "streaks") were first observed by Robert Hooke in 1665 using a large convex lens and two candles.  How it works

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