Schlieren Photograph
Density variations in air caused by air being cooled (left) and warmed (right) by a goblet filled with cold or hot water cause light rays passing through these regions to be obstructed by a "knife edge" in a Schlieren beam and thus appear darker as contrasted to a uniform density region or to be moved away from the knife edge and thus add illumination to the base density associated with a region of uniform density. This allows us to visualize convection currents in air caused by temperature differences in the subject field.
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