There is a physical phenomenon called Rayleigh scattering that causes light to scatter when it passes through particles that have a diameter one-tenth that of the wavelength (colour) of the light. Sunlight is made up of all different colours of light, but because of the elements in the atmosphere the colour blue is scattered much more efficiently than the other colours.

When you look at the sky on a clear day, you can see the sun as a bright disk. The blueness you see everywhere else is all of the atoms in the atmosphere scattering blue light toward you. Because red light, yellow light, green light and the other colours aren't scattered nearly as well, you see the sky as blue. And that is why the Sky is blue - Simples!

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