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Why the sky is blue

Sunlight appears white to the human eye but is a mixture of all the colors of the rainbow, each distinguished by their different wavelength. When the sun’s light reaches the Earth’s atmosphere it is scattered, or deflected, by tiny molecules of gas in the air. Because these molecules are much smaller than the wavelength of visible light, the amount of scattering depends on the wavelength.

When the Sun is close to the horizon, at sunset or sunrise, light from the low Sun has to pass through more atmosphere and most of the blue light has already been scattered leaving just the red. The result is the beautiful colours seen at sunset and sunrise and, very occasionally, a flash of green light.

When the air is clear the sunset will appear yellow, because the light from the sun has passed a long distance through air and some of the blue light has been scattered away.  If the air is polluted with small particles, natural or otherwise, the sunset will be more red.

Blue Haze and Blue Moon

Clouds and dust haze appear white because they consist of particles larger than the wavelengths of light, which scatter all wavelengths equally (known as Mie scattering).  But sometimes there might be other particles in the air that are much smaller.  Some mountainous regions – the Bliue Mountains of Australia – are famous for their blue haze – from the eucalyptus forests.  Aerosols of terpenes from the vegetation react with ozone in the atmosphere to form small particles about 200 nm across, and these particles scatter the blue light.  A forest fire or volcanic eruption may occasionally fill the atmosphere with fine particles of 500–800 nm across, being the right size to scatter red light.  This may cause the moon to have a blue tinge since the red light has been scattered out; a rare phenomenon, occurring literally once in a blue moon.

Light travels in a straight line unless something gets in the way to

  • reflect it (like a mirror)
  • bend it (like a prism)
  • or scatter it (like molecules of the gases in the atmosphere)

Since scattering by the atmosphere causes the sky to be blue,

a planet with no atmosphere cannot have a bright sky.

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