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.
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