A green flash can be seen at sunrise and sunset when part of the sun suddenly changes color, lasting for only a second or two. This is because at sunset, the refractive delay of the sunset is usually a second or two longer for blue and violet wavelengths than for red. In general, the red image of the sun disappears first, followed by yellow, green, blue and violet. Violet isnít the last color to be seen at sunset because the shortest wavelengths are scattered the most strongly. At the horizon, the path length through the air is very long, and the shortest wavelengths are almost completely removed. Usually there is enough haze in the air that violet and blue are completely removed, so that green is the last color seen at sunset, or the first at sunrise. This image of a green flash was taken at sunset on April 30, 1995 near Cabo San Lucas, Mexico.
Image © 1995 Kenneth D. Langford