It’s known as Schlieren Optics, and it’s basically an optical technique that allows you to see how air moves. According to the Harvard Natural Sciences Lecture Demonstrations, it works something like this… 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.
In the demonstration, they use a candle flame, hair dryer, helium gas, and sulfur hexafluoride gas. If you’ve never seen something like this before, prepare to be amazed… it’s quite incredible.
This video below explains more in-depth how it works
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