Light shown to be Doppler shifted by reflecting off rotating objects

Science : The Doppler effect is the familiar change in frequency or pitch of light and sound waves as a result of the motion of the source relative to an observer. The linear version of this effect has been known since the 1800s, but in the 1980s and 1990s, it was determined that rotating sources of polarized light can demonstrate Doppler shifts as well. One of the other effects is that rotating light sources could give the emitted light orbital angular momentum (OAM)—a rotation in the light’s electric and magnetic fields. Now, Miles Padgett and Martin Lavery of the University of Glasgow in the UK and their colleagues have shown that light that is reflected off a rotating surface is also Doppler shifted. They fired a laser at a plastic rotor, then used detectors to collect and filter the scattered light waves into two signals—those with positive (clockwise) OAM and those with negative (counterclockwise) OAM. They found that light with positive OAM experienced an increase in frequency, and light with negative OAM experienced a decrease in frequency. The magnitude of the changes allowed the researchers to calculate the rotation speed of the rotor. Padgett suggests that the technique could be used to measure the rotation of distant planets or of air currents at wind farms.