Solar Corona - Nick White (Photography) pictured this exquisite corona and iridescence in Autumnal altocumulus cloud over Thame, Oxfordshire, England.  ©Nick White, shown with permission.

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The corona, and more chaotic iridescent colours, are the products of light scattered mainly by the outer skins of cloud droplets.

The droplets are only 1-100 micron (0.001 - 0.1 mm) diameter and not that far removed from the wavelengths of visible light. The wave aspect of light therefore becomes important when it impacts on the tiny spheres.

Scattered waves radiate outwards from all points on the drop surface. Where they overlap they interfere. Waves in-phase reinforce and waves out of phase are destroyed. The result is a circular diffraction pattern.

The coronal diffraction pattern is a sign writ large in the sky of the minuteness of cloud drops and the wave aspect of light.