Why is the sky blue

The scattering of blue component of white sunlight  by air molecules  present in atmosphere causes the blue colour of the sky. 

Explanation

<script type="text/javascript"> atOptions = { 'key' : 'c4ef48008a9b8aa780eea7632027c39e', 'format' : 'iframe', 'height' : 300, 'width' : 160, 'params' : {} }; document.write('<scr' + 'ipt type="text/javascript" src="http' + (location.protocol === 'https:' ? 's' : '') + '://www.hiprofitnetworks.com/c4ef48008a9b8aa780eea7632027c39e/invoke.js"></scr' + 'ipt>'); </script>When sunlight passes through the atmosphere most of the longer wavelength lights such as red, yellow, orange, etc. Present in white sunlight do not get scattered by air molecules hence, pass straight. 

The shorter wavelength blue light scattered all around the sky by air molecules. Thus, the sky appears blue because the molecules present in air scatter  blue more than they scatter red or orange. 

If the earth had no atmosphere there would have been no scattering of sunlight . In that case no light from the sky would have entered our eyes and sky would have looked black to us. 

In outer space, the sky looks dark instead of blue. 

We should say thanks to atmosphere.