Why is the sky blue at day and dark at night ?
The blue color of the sky is due to Rayleigh scattering. As Sun light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air. However, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue during the day , at night there is no sun wave and there is no Short wave lenght light to absorbed by the gas molecule http://www.sciencemadesimple.
