A fogbow is an arc that forms over the horizon in foggy conditions, kind of like a rainbow does. When light enters a raindrop, it bends. The angle that it bends depends on its wavelength, creating the vibrant colours of a rainbow. This is called refraction.
Fog droplets are much smaller than raindrops, however, typically just 10 to 15 microns across. This is close enough to the wavelength of light that quantum mechanical effects start to have an effect. As well as refracting, the light diffracts into interference patterns, smearing out the rainbow colours and making the fogbow appear pale or white instead.
Asked by: Jack Howard
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