Sunset Glory ~ Reddened sunset rainbows are well known - Are they?    Here we have a rarer sight, a glory glowing with soft pink hues from the high altitude sun’s setting rays.    There are hints of three rings perhaps more apparent for their monochromaticity.

This subtle sight was seen by Shelley Banks (Prairie Nature, Writing Editing Photography) on a journey from Vancouver to Regina, Canada.

All images ©Shelley Banks, shown with permission
The more usual appearance of a glory pictured again by Shelley Banks from a prop plane east of Calgary

Glories are directly opposite the sun, a diffraction pattern from scattering by small (1-50 micron diameter) cloud droplets.

The classical ray-path description at left is inadequate, indeed impossible.   A water drop cannot send rays directly backwards.  

Our macroscopic experience does not encompass the behaviour of light waves constricted in a tiny spherical cavity but our mathematics does.    Dutch physicist/chemist and Nobel prize winner Peter Debye developed a light scattering formulation that, unlike Mie theory, provides some information on the mechanism of light scattering in droplets.  

Debye theory shows that the major source of the glory's central illumination is light reflected once inside droplets. There are lesser contributions from light reflected 10, 6 and 5 times.    Surface waves also play a role.


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