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Asked in GSEB Board July 2016 · Dispersion and scattering
Given: the colour of the daytime sky away from the sun.
Idea: air molecules are far smaller than the wavelength of light, so they scatter it by Rayleigh scattering, whose strength goes as 1/(λ⁴).
Blue light, λ≈ 450 nm, has a shorter wavelength than red, λ≈ 650 nm, so it is scattered about ((650)/(450))⁴≈ 4 times more strongly.
Light reaching the eye from a patch of blank sky is scattered light, so it is rich in blue.
The same effect seen the other way round reddens the sun at sunrise and sunset, when the blue has been scattered out of the long slanting beam.
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