What is "schlieren photography"

n. Any of several techniques used to record regions of different refractive index in a fluid; especially used to visualize the flow of air around objects

Schlieren photography is a visual process that is used to photograph the flow of fluids of varying density. Invented by the German physicist August Toepler in 1864 to study supersonic motion, it is widely used in aeronautical engineering to photograph the flow of air around objects.