Rotational Doppler Effect

Angelica M. Cortes Velez

The article “Like a speeding watch” is about the rotation in lights electric field vector, that can alter the lights frequency. It has been spotted in the laboratory, that this rotational is the equivalent of the Doppler effect. The author of the article was, Miles Padgett, his report 

“Physical Review Letter”, with Barreiro and colleagues have measured the broadening of a spectral line caused by an effect analogous to the conventional Doppler Effect.

Talking more about Miles Padgett, him is a Professor of Optics in the Department of Physics and Astronomy in the University of Glasgow. He heads a 15-person team covering a wide spectrum from blue-sky research to applied commercial development, funded by a combination of government charity and industry. In 2009 Padgett was awarded the Institute of Physics, Young Medal "for pioneering work on optical angular momentum".

Padgett has an international reputation for his contribution to the fundamental understanding of light's momentum, including conversion of optical tweezers to optical spanners, observation of a rotational form of the Doppler shift and an angular form of Heisenberg's uncertainty principle.

The Doppler shift or Doppler effect is the change in frequency of a wave for an observer moving relative to the source of the wave. Doppler first proposed the effect in 1842 in his treatise “ On the coloured light of the binary stars and some other stars of the heaven”. The hypothesis was tested for sound waves by Buys Ballot in 1845. The Doppler effect is used in some types of radar, to measure the velocity of detected objects.

The shift in frequency of this case arises from rotation. The effect was observed in terms of Jones polarization matrices, wich describe this effect in a medium with an orientation of the electric field. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding. 

Polarization (also polarisation) is a property of certain types of waves that describes the orientation of their oscillations. Electromagnetic waves, such as light, and gravitational waves exhibit polarization; acoustic waves (sound waves) in a gas or liquid do not have polarization because the direction of vibration and direction of propagation are the same. By convention, the polarization of light is described by specifying the orientation of the wave's electric field at a point in space over one period of the oscillation. When light travels in free space, in most cases it propagates as a transverse wave—the polarization is perpendicular to the wave's direction of travel. In this case, the electric field may be oriented in a single direction (linear polarization), or it may rotate as the wave travels (circular or elliptical polarization). In the latter cases, the oscillations can rotate either towards the right or towards the left in the direction of travel. Depending on which rotation is present in a given wave it is called the wave's chirality or handedness. In general the polarization of an electromagnetic (EM) wave is a complex issue. For instance in a waveguide such as an optical fiber, or for radially polarized beams in free space, the description of the wave's polarization is more complicated, as the fields can have longitudinal as well as transverse components. Such EM waves are either TM or hybrid modes.

In many areas of astronomy, the study of polarized electromagnetic radiation from outer space is of great importance. Although not usually a factor in the thermal radiation of stars, polarization is also present in radiation from coherent astronomical sources (e.g. hydroxyl or methanol masers), and incoherent sources such as the large radio lobes in active galaxies, and pulsar radio radiation (which may, it is speculated, sometimes be coherent), and is also imposed upon starlight by scattering from interstellar dust. Apart from providing information on sources of radiation and scattering, polarization also probes the interstellar magnetic field via Faraday rotation. The polarization of the cosmic microwave background is being used to study the physics of the very early universe. Synchrotron radiation is inherently polarised. It has been suggested that astronomical sources caused the chirality of biological molecules on Earth. The rotational Doppler effect is interesting because here the spin and orbital components are indistinguishable. Instead the total angular momentum of the light beam, that is crucial.