When light passes between areas of different air density, it bends. You've probably noticed the way distant pavement seems to shimmer on a hot day, or the way stars appear to twinkle. You're seeing light that has been distorted as it passes through varying air densities, which are in turn created by varying temperatures and pressures.
In the mid-19th century, German physicist August Toepler invented a photography technique called Schlieren Flow Visualization to visually capture these changes in density. The setup is a bit hard to explain in words (watch the video above for a full explanation) but it allows scientists and engineers to see things that are normally invisible: the rising heat from a candle, the turbulence around an airplane wing, the plume of a sneeze.
It can also be used to see sound. Sound, after all, is just another change in air density — a traveling compression wave. A speaker pushes on the surrounding air, creating a wave that travels outward until it encounters the ear drum.
As a result, Schlieren Flow Visualization can be used to see sound as well. Sound waves travel extremely fast (761.2 miles per hour) so a high-speed camera is needed to see the waves' passage.
Michael Hargather, a professor of mechanical engineering at New Mexico Tech, uses just this approach to study the shock wave caused by explosives. He's made Schlieren videos of everything from firecrackers to car bombs, but he's also filmed more benign shock waves — a book landing on a table, the end of a towel being snapped, and a man clapping. You can see them all in the video.
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