Why is the colour of the clear sky blue?
Ans.
This is due to the Rayleigh scattering of sunlight.
Amount of scattering is inversely proportional to fourth power of wavelength.
Wavelength of blue colour is the lowest so blue colour is scattered the most.
The molecules of air and other fine particles in the atmosphere have smaller size than the wavelength of visible light. These particles are more effective in scattering light of shorter wavelengths (blue) than light of longer wavelengths (red).