Scattered Light Makes The Sky Blue

Question: Why is the sky blue?

Answer: The color of our sky is no longer a scientific puzzle. Today we know that the blue color is formed by scattered light. When the bright white sunlight hits widespread molecules such as oxygen and nitrogen in the air, it is scattered and produces all colors. The blue color has a shorter wavelength compared to all the other colors and is therefore scattered more.

This also explains the orange color you see at sunset. 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.

See also:

What Causes Lightning To Be Colored?

The first steps towards correctly explaining the color of the sky were taken by John Tyndall in 1859.  He discovered that when light passes through a clear fluid holding small particles in suspension, the shorter blue wavelengths are scattered more strongly than the red.

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Source:

University of California