Scientists in a spin over Doppler Shift discovery

Scientists at the Universities of Glasgow and Strathclyde have discovered rotational speed can be determined by measuring Doppler Shift – the same effect utilised in radar speed guns.

Doppler Shift is a phenomenon everyone is aware of, if perhaps not by name, and is most often experienced by the sound of a siren from a police car or ambulance rising and falling in pitch as it passes by.

It is a result of the frequency change due to the position of the observer relative to the source, so each sound wave from an approaching siren is generated a little closer to the observer each time and a little further away as it travels away.

The effect is utilised in radar speed guns, when the waves generated by the gun reflect off a car, the change in frequency is detected and speed calculated.

Both of these examples concern linear motion – objects travelling along a straight path – but scientists at the University of Glasgow have discovered Doppler Shift can be used to detect rotational motion too, but only when using ‘twisted light’.

Researchers in the School of Physics & Astronomy used a beam of light that had ‘angular momentum’ – or spin – and pointed it at rotating objects.

Even though the light hit the object head-on, because the light was twisted like a corkscrew, it was actually hitting the object at a slight angle. The light is scattered and a Doppler Shift is observed. Light without this spin would see no such shift.

Martin Lavery, research assistant who conducted the experiments, said: “Detecting changes in wave frequency cased by the Doppler Effect is a well established method of measuring the speed of an object travelling along a linear path.

“In our study, we wanted to look at this effect for rotating objects, for example where it could be used to detect speed by looking at the spinning wheels of a car.

“This research is the first realisation that there is an angular equivalent to the Doppler Effect that allows rotational speed to be determined head-on when using twisted light beams.

“The ability to determine rotational speed has many useful applications, such as in aircraft engine manufacturing or in preventing damage to wind turbines by measuring the velocity of vortexes in the air.”

The research was undertaken by Professor Miles Padgett and Mr Lavery at the University of Glasgow and Professor Stephen Barnett and Dr Fiona Speirits at the University of Strathclyde.

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