Why does the sky appear blue on a clear day ?

The scattering of blue component of white sunlight by air molecules presents in the atmosphere cause the sky to look blue during a clear day. When sunlight passes through the atmosphere, most of the longer wavelength lights do not get scattered much and hence pass straight through the atmosphere. The shorter wavelength blue light is, however, scattered all around the sky and whichever direction we look, some of this scattered blue light enters our eyes.