The green flash is the phenomenon that the last bit of the sun colors green when the sun sets below the horizon. The effect is due to atmospheric refraction of light. The refraction angle for the green end of the light spectrum (the shorter wavelengths) is slightly larger than the red part. As a result, the spectrum of colors of the sun is spread over a small vertical distance (a few sec. of arc). So, the top rim of the sun appears green (blue in extremely clear conditions) and red on the lower rim. When the sun sets, the green rim is the last to disappear. During sunrise, the effect can also be seen, but this is a lot harder because one does not know exactly where to look. The actual green flash, a green flame above the point where the sun set below horizon a few seconds after sunset, is extremely rare. However, the green rim can frequently be seen, even if the sun is well above the horizon, as well as small green flashes due to inversions in the atmosphere. Such an inversion may refract the green rim and separate it for a few seconds off the solar disk. Such green flashes are relatively common yet hard to see, even with the aided eye.
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