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Corona : An Optical Marvel

New Delhi, June 24 (G’nY News Service): In meteorology, a corona is produced by the diffraction of light from either the Sun or the Moon by individual small water droplets and sometimes tiny ice crystals of a cloud( or on a foggy glass surface). A corona consists of small numbers of concentric colored rings around a celestial object and a central bright aureole. The angular size of the corona depends on the diameters of the cloud droplets – small droplets produce large coronae. For the same reason, the corona is clearest when the size of the droplets is most uniform. Coronae differ from haloes; the latter are formed by refraction (rather than diffraction) from comparatively large rather than small ice crystals.

The corona is the result of scattering of light by particles ranging in size from about 10 μm to 100 μm. In nature, these particles can be ice needles or cloud droplets. Corona appears as rings of colored light surrounding the luminous source, usually the moon or the sun. These rings are not as large as the rings of the halo, and are usually more strikingly colored.

The classic corona consists of a bright aureole, bluish in the center and brownish on the periphery, surrounded by one or more rings of lesser intensity that are bluish on the inside and red on the outside, passing through green and sometimes yellow on the way. Usually only one ring is visible, but up to three rings have been observed.


Although Corona occurs around the sun often, Corona is usually observed around the moon, since the moon is easy to look at. Since looking at the sun is uncomfortable, it is necessary to screen off its brilliance when looking for the corona. Welder’s goggles can be used. The problem here is the same as in viewing eclipses, and the same cautions are necessary.

Compared to the rainbow and the halo, the corona is a relatively neglected part of meteorological optics. However, it is one of the most frequently observed phenomena of this type, and can furnish intellectual challenge, useful information and visual pleasure.


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