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.
