Blue Color of Sky:

The blue colour of the sky is due to the scattering of light by small particles of the atmosphere. When light is incident on particles, whose size is smaller than the wavelength of light, it is scattered. According to Rayleigh, the percentage of light scattered is inversely proportional to the fourth power of the wavelength. Wavelength of the blue colour is smaller as compared to the wavelength of red colour. Therefore, blue colour is scattered more as compared to the red colour. When white light from the sun falls on the particles of the atmosphere, the blue colour is scattered and if we look overhead, the shy, far away from the sun appears white because we get direct light from the sun. At sunset or sunrise, the sun and the neighbouring portion appears red because sunlight travels a greater part of the earth's atmosphere and reaches directly the observer's eye. This light is deprived of the blue colour due to scattering and the remaining colour (white minus blue) is red. Also near sunset sunlight reflected from clouds near the earth's surface appears red because the light incident on the cloud is rich in red colour (blue colour being removed by scattering).

It is to be remembered that the sky will appear black in the absence of earth's atmosphere because no colour is scattered in that case. While flying in an aeroplane, one can observe the sky to be black at high altitudes.