Diffraction halo

 

Simulated double halo with a mountain foreground.

A diffraction halo is formed when light (sunlight) shines through a vast number of randomly distributed small obstacles of uniform size (water droplets). The angular radius is wavelength dependent and the effect is analogous to diffraction through a tiny hole. The angular radius is given by...

q(rad) = 1.22 l/a

... where a is the diameter of the droplets and l is the wavelength of the light.

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