Rotational Doppler shift spotted in twisted light
"Twisted light" has been used by researchers in the UK to develop a new way of measuring the angular velocity of a remote spinning object. The team fired two beams of light carrying orbital angular momentum at a rotating surface and showed that the resulting interference pattern in the reflected light is related to the surface’s angular velocity. The researchers hope that the phenomenon can be used to develop systems to carry out a range of practical measurements, from monitoring industrial equipment to calculating rotation rates of astronomical objects.
The Doppler shift – a shift in the frequency of waves emitted or reflected by an object moving relative to the observer – is a well-understood phenomenon with numerous uses in science and engineering. These include determining the speed at which distant galaxies are approaching or receding and making it easier for the police to catch speeding motorists. It can also be used to study objects that are rotating when some of the object is rotating towards the observer and some is rotating away. However, it cannot be used to work out how fast an object is rotating about the axis pointing along the direct line of sight between the object, light source and observer.