Glory (optical phenomenon)
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A glory is an optical phenomenon produced by light backscattered (a combination of diffraction, reflection and refraction) towards its source by a cloud of uniformly-sized water droplets. A glory has multiple colored rings. The angular size is much smaller than a rainbow, about 5° to 20°, depending on the size of the droplets. Since it is seen in the direction opposite the sun, it is most commonly observed while airborne, with the glory surrounding the airplane's shadow on clouds (this is often called The Glory of the Pilot). The phenomenon is also known as an anthelion (plural anthelia, from late Greek ανθηλιος, meaning "opposite the sun").
In 1947, the Dutch astronomer Hendrik van de Hulst suggested that surface waves were involved in the formation of glories. The colored rings of the glory are caused by two-ray interference between "short" and "long" path surface waves – which are generated by light rays entering the droplets at diametrically opposite points (both rays suffer one internal reflection). For a more complete description, see the external link to "How are glories formed?"
Glories are often seen in association with a Brocken spectre, the apparently enormously magnified shadow of an observer cast, when the Sun is low, upon the upper surfaces of clouds that are below the mountain upon which he stands. The name derives from the Brocken, the tallest peak of the Harz mountain range in Germany. Because the peak is above the cloud level, and the area is frequently misty, the condition of a shadow cast onto a cloud layer is relatively favored. The appearance of giant shadows that seemed to move by themselves due to the movement of the cloud layer (this movement is another part of the definition of the Brocken Spectre), and which were surrounded by optical glory halos, may have contributed to the reputation the Harz mountains hold as a refuge for witches and evil spirits. In Goethe's Faust, the Brocken is called the Blocksberg and is the site of the Witches' Sabbath on Walpurgis Night.
C. T. R. Wilson saw a glory while working as a temporary observer at the Ben Nevis weather station. Inspired by the impressive sight, he decided to build a device for creating clouds in the laboratory, so that he could make a synthetic, small-scale glory. His work led directly to the cloud chamber, a device for detecting ionizing radiation for which he and Arthur Compton received the Nobel Prize for Physics in 1927.
In China, this phenomenon is called Buddha's light (佛光). It was often observed on cloud-shrouded high mountains, such as Huangshan Mountains and Mount Emei. Records of the phenomenon at Mount Emei date back to A.D. 63. The colorful halo always surrounds the observer's own shadow, and thus was often taken to show the observer's personal enlightenment (associated with Buddha or divinity) until modern science explained the optics behind the phenomenon.
