Why Does the Sunny Sky Look Blue?

In the sky there is a very thick layer of air which is colourless. So why does the sky look blue？ It has something to do with the irradiation of sunlight.

　　Sunlight is formed by the seven colours of red， orange， yellow， green， blue， indigo-blue and purple. When the sunlight shines on the air and meets the dust and vapour in the air， the blue light in the sun-light immediately radiates¹ to all directions， making the sky look blue. It is very easy for the yellow and red colours in the sunlight to go through the thick layer of air and then shine over the earth. Therefore， the sunlight looks yellow from the ground.