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what about the colour of the sky is blue
Sky appears blue to  the process of  scattering of sunlight. When sunlight enters the earth atmosphere, air and water vapour molecules will absorb part of the light and reradiate it to all directions. This is called scattering. White sunlight is composed of light waves of different colors, whereas blue light has the shortest wavelength and red light has the longest. The short-wavelength blue light is easier to be scattered. The sun is very close to the horizon during sunset. Sunlight must pass through a thicker atmosphere in order to reach the ground. Most of the blue light has been scattered away and red light remains.  On the other hand, sunlight passes through a thinner atmosphere during daytime, blue light is scattered less. Therefore the sun appears white. At the same time the sky is full of scattered blue light, that is why the whole sky appears blue.
 It was explained 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.    The scattered light can also be shown to be polarised using a filter of polarised light, just as the sky appears a deeper blue through polaroid sun glasses.
This is  called the Tyndall effect, but it is more commonly known to physicists as Rayleigh scattering—after Lord Rayleigh, who studied it in more detail a few years later.  He showed that the amount of light scattered is inversely proportional to the fourth power of wavelength for sufficiently small particles.  It follows that blue light is scattered more than red light by a factor of (700/400)4 ~= 10.


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