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Doppler effect takes a spin

Light's twistiness allows researchers to measure rotating object's speed

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7:14am, August 2, 2013

A twist on the physics that cops use to clock speeding drivers can determine how fast an object is spinning. The approach could be used to protect wind turbines from damaging winds, learn about distant astronomical objects or detect tornadoes.

The Doppler effect is familiar to anyone who has heard the pitch of a siren rise and fall as an ambulance whizzes past. It describes how the frequency of light waves (or the pitch of sound waves) emitted by a moving object becomes higher as the object approaches an observer and lower as it moves away. Today people exploit the Doppler effect to track the motion of raindrops, cars and galaxies.

But Doppler’s kryptonite has always been rotating objects that are neither moving toward nor away from the observer, like the second hand on a clock.

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