Stories from Physics for 11-14 14-16
The green flash is a rare optical phenomenon visible at sunrise and sunset. Under the right conditions, just as the Sun is about to disappear beneath the horizon, the last visible piece of its rim may shine with a green light.
The phenomenon can be understood by imagining the Sun to consist of a series of overlapping discs of different wavelengths. When it is overhead, the discs align exactly and the Sun appears yellow. When the Sun is lower in the sky, atmospheric refraction displaces the discs by different amounts, and as the Sun sinks below the horizon, the last discs to be visible are the green and blue/purple discs. However, as much of the blue light is scattered by the atmosphere, the final disc to be seen is the green one, registering as a green flash.