Probing the 'Glory Ring' Mystery

Ever notice bright, colored rings around a distant airplane? Researchers have described this phenomenon as a strong backscattering of light by droplets of water. While the theory was well-developed, it remained difficult to verify experimentally.
More than 50 years ago, H. Van de Hulst explained "glory" scattering in terms of waves traveling around the surface of each droplet. Each water particle radiated weak wavefronts in the direction of the observer.
A group of researchers from Oklahoma State University in Stillwater fired electromagnetic pulses of 0.1 THz with a spectral peak at a wavelength of 1 mm at a 6.35-mm-diameter aluminum sphere. A short optical pulse from a laser directed at a dipole antenna generated the radiation. Another optoelectronic system, illuminated by a delayed laser pulse, allowed time-delayed reception of the scattered signal. By changing the time of delay, glory rays were separated from direct reflections. The scientists found the spectrum of the glory rings was consistent with theoretical models.