Shadowgraph is an optical method that reveals non-uniformities in transparent media like air, water, or glass.
The plume of hot air rising from a fire, for example, can be seen by way of its shadow cast upon a nearby surface by the uniform sunlight.
It is used in aeronautical engineering to see the flow about high-speed aircraft and missiles, as well as in combustion research, ballistics, explosions, and in the testing of glass.
In the shadowgram, the differences in light intensity are proportional to the second spatial derivative (Laplacian) of the refractive index field in the transparent medium under study.
One first use was made by Disney Studios on the Three Blind Mouseketeers (1936) in the series of animated short films Silly Symphonies.