why actually sky is blue? If dispersed blue ray is hitting ground then why we can't see that blue colour on ground?
Scattering of light is the phenomenon responsible for the blue colour of the sky.
The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of visible light. These are more effective in scattering light of shorter wavelengths at the blue end than light of longer wavelengths at the red end. The red light has a wavelength about 1.8 times greater than blue light. Thus, when sunlight passes through the atmosphere, the fine particles in air scatter the blue color (shorter wavelengths) more strongly than red. The scattered blue light enters our eyes and sky appears to be blue.
Rayleigh scattering means the shorter wavelengths will be scattered much more than longer wavelengths, which explains why the sky isn't red or green.