green flash or emerald flash, a refractive phenomenon of the atmosphere where the top edge of the setting (or, less frequently, rising) sun will momentarily turn emerald green. The green color lasts from a fraction of a second to two seconds. It is usually seen over a low distant horizon, such that as of the ocean or a prairie, when the sky is clear and free of clouds. The phenomenon was explained by James Prescott Joule in a letter to the Manchester Literary and Philosophical Society in 1869 and popularized in Jules Verne's novel Le Rayon Vert (1882; tr. The Green Ray, 1883).
The green flash occurs primarily because the atmosphere acts like a weak prism, refracting sunlight and separating it into different colors. As the sun sets, red and orange light, which are refracted the least, disappear first. Although green light is in roughly the middle of the spectrum, it is usually the last color to be seen by someone watching a sunset because blue and violet light are practically all absorbed by the contamination in the atmosphere, which scatters blue light and removes it from the line of sight. Under extraordinary conditions, however, a "blue flash" may be seen. At sunrise the phenomenon is reversed, with the green flash appearing first.