What Causes the Green Flash?
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The green flash is possible due the fact that the air is both a refractive medium ( it bends light) and it is dispersive (it separates white light into colors due to the different wavelengths of visible colors). The refractive property of the air causes the image of the sun to be elevated beyond its true position. The dispersive property of the air causes the white light from the sun to separate into its components of color (similar to that of a rainbow) so that the blue light (highest wave length of visible light) is concentrated in a higher arc than the red light ( lowest wavelength of visible light.). As the sun sinks below the horizon, the arcs of the different colors are subtracted from the image ( red, orange, yellow ect) until all that remains for a split second is the green, and then the blue arc, before the sun slips below the horizon.