A flash of green light with the setting sun?

Ask Tom Why

Dear Tom,
I have been told the setting sun interacts with the atmosphere to produce a green flash, but I’ve never seen one. Does it really happen?
Richard Butler Winthrop Harbor
Dear Richard,
It’s rare, but it does happen and is definitely real. It is a momentary flash of green light emanating from the upper rim of the rising or setting sun as it breaks the horizon. Widely thought to be visible only in the tropics, the green flash can actually be seen anywhere with an unimpeded view of the horizon. Sunlight passing through the atmosphere is bent upward, and the bending is greatest when the sun is on the horizon. The shortest wavelengths of light (blue and green) are bent the most so they are the first colors seen on the rising sun and the last as it sets. The blue wavelengths are removed by atmospheric scattering, leaving only the green visible for the flash.


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