the adventures of teapot the cat

Shock Wave Imaging

Posted on January 11th, 2006

American Scientist Online has an amazing article on High-speed Imaging of Shock Waves, Explosions and Gunshots, similar to the pics and video referenced in an earlier post.

Even a transparent phenomenon sometimes leaves telltale signs. For shock waves these signs can include moisture condensation, dust disturbance, whitecaps on water, optical distortion and shadows. Certain aquatic predators find their transparent prey by the shadows that the Sun casts on the ocean floor.

Robert Hooke discovered this effect more than three centuries ago while observing the shadow of a burning candle cast by sunlight. Above the flame he saw a plume of hot air that was not directly visible but cast a shadow because the heat changes the density of the air, which refracts light rays. What Hooke described is now called the “shadowgraph” method, and it’s a simple approach that works extremely well for visualizing shock waves.

Hooke also discovered another visible trait of transparent phenomena: They can distort the features of a background pattern that is viewed through them. In this way an antique glass windowpane warps one’s view of the world outside. But Hooke was ahead of his time, so this observation principle lay unused until the mid-19th century, when the German scientist August Toepler rediscovered it and used it to observe electric sparks. He saw spherical waves in the air from loud spark discharges and thought he was observing sound, but actually he was the first to see shock waves. Toepler named his optical method the Schlieren method (Schlieren means “streaks” in German). Although the technology has changed significantly, particularly for capturing large fields of view, that name for this method persists today.

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