Why the sky is blue?

A clear cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light. When we look towards the sun at sunset, we see red and orange colours because the blue light has been scattered out and away from the line of sight. The white light from the sun is a mixture of all colours of the rainbow. This was demonstrated by Isaac Newton, who used a prism to separate the different colours and so form a spectrum. The colours of light are distinguished by their different wavelengths. The visible part of the spectrum ranges from red light with a wavelength of about 720 nm, to violet with a wavelength of about 380 nm, with orange, yellow, green, blue and indigo between. The three different types of colour receptors in the retina of the human eye respond most strongly to red, green and blue wavelengths, giving us our colour vision.

According to me, it is due to refecltion of water. As we know , we have 75% of water on earth. So when sun shines bright the color of water is reflected and sky appears bule. Similary after sun set , the sky turns black and as there is no light, there is no refelction.

look at these sites you may find them helpful: http://www.sciencemadesimple.com/sky_blue.html http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html