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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,
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, 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,
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, 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.
Physicist Martin Lavery at the University of Glasgow in Scotland and colleagues thought that they could measure rotational speed by determining how a spinning object changes light waves’ orbital angular momentum. In focused beams such as lasers, successive waves of light move like ocean waves on a beach: straight on,
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, with periodic crests and troughs. But most light is messier: The waves wind around like a corkscrew.
Lavery’s team plated a plastic disk with aluminum foil and hooked it up to a spinning motor. Then the researchers bounced a light beam off the disk that had a special property: Its waves twisted either clockwise or counterclockwise,
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.
When Lavery’s team measured the light after it bounced off the disk,
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, they found that the frequency of waves twisting in the same direction as the disk was spinning had become higher,
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, while the light twisting in the opposite direction had a lower frequency. The change allowed the researchers to calculate the disk’s speed of rotation,
they report in the Aug. 2 Science,
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.
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