Green flash is a rare phenomena which appears as a spot or slit of green light that is briefly visible at the top of the sun's disk, usually at sunset. The flash can occur at sunrise too, although it is shorter in duration and more bluish in colour. The green flash is an atmospheric refractive phenomenon caused by the bending of sunlight through the layer of air immediately above the horizon. When light refracts, it breaks down into the colours of the spectrum. The green band, magnified by the atmosphere, appears for a fraction of a second or longer at the top edge of the sun.