Our earth is surrounded by a blanket of air, which is called the atmosphere. It is mainly composed of nitrogen, oxygen, carbon dioxide, dust particles, water vapours and other gases. The light that comes to us from the sun has to travel through this atmosphere.
Sun is the main source of light for earth. When the sunlight passes through the atmosphere, it is scattered by the dust particles, water and air molecules in all the directions. This is why, there is light all around after the sunrise.
We know that the sunlight is composed of seven colours: violet, indigo, blue, green, yellow, orange and red. When the sunrays pass though the atmospheric molecules, the violet, indigo and blue colours get reflected most and the red colour least of them. Therefore, when we look at the sky the light that enters our eyes mainly consists of violet, indigo and blue colours. The mixture of these three colours is almost blue. That is why the sky appears blue.
If the earth had no atmosphere, the sky would have just appeared dark. This fact becomes very clear when we look at the sky from the surface of the moon. It appears dark since there is no atmosphere in the moon. Similarly, the sky appears dark when we look from a spacecraft, because there are no light scattering particles in the higher space.
