The molecules of air and other fine particles in the atmosphere have a size smaller than the wavelength of visible light. Thus, they are more effective in scattering light of shorter wavelengths at the blue end than light of longer wavelengths at the red end. Red light has a wavelength greater than blue light. Thus, when sunlight passes through the atmosphere, the fine particles in air scatter blue colour (shorter wavelengths) more strongly than red. The scattered blue light enters our eyes.