What is the blue sky?
The sky is blue mostly during the day due to a phenomenon called Raleigh scattering, which refers to the scattering of electromagnetic radiation by particles of a much smaller wavelength. Above the sky, the molecules of air and the other fine particles in the atmosphere are smaller than the wavelength of visible light. This scattered blue light enters our retina and makes the ocean and sky appear blue.
Reason
The rays of sunlight are scattered by the fine particles found in the atmosphere that come through down to earth called the diffuse sky radiation, and though only about 1/3rd of light is scattered, the smallest wavelengths of light tend to spread more easily. These shorter wavelengths correspond to blue hues, so when we look at the sky, it all looks blue (or maybe a lighter blue shade). The angle at which the sunlight ray enters the atmosphere is significantly changed during the sunsets and sunrises. Most of the blue and green (shorter) wavelengths of light are scattered even before reaching the lower atmosphere, which is why we can see more orange and red colors in the sky and other colors.