PerspectivePhysics

Spinning the Doppler Effect

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Science  02 Aug 2013:
Vol. 341, Issue 6145, pp. 464-465
DOI: 10.1126/science.1242097

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Summary

The sound from a moving object undergoes a pitch change—the Doppler effect—caused by the acoustic wave frequency increasing as the object approaches and decreasing as it moves away—A similar effect—a blue or red color shift—occurs for light from moving astronomical objects. Waves scattered off a moving object are also subject to a Doppler shift, an effect used in sonar and radar systems to deduce speed. A spinning object may also induce a Doppler effect in backscattered waves if the rotation axis is not pointing directly at the wave source and detector. The spinning object can thus be seen, at any given time, as composed of parts that are moving toward the detector and parts that are receding from it. However, if the rotation axis points toward the detector, this “piecewise” Doppler effect vanishes. This limitation can now be overcome by using “twisted” light. On page 537 of this issue, Lavery et al. (1) demonstrate that a spinning object with an optically rough surface may induce a Doppler effect in light reflected parallel to the rotation axis, provided that the light carries orbital angular momentum (OAM).