Why is the sky blue in colour?

The sky looks blue in colour due to the scattering of lights. 

Explanation: The light rays emitted by the sun passes through the earth's atmosphere. As the earth's atmosphere consists of various gases and particles. When the light ray emitted by sun collide with those particles which are present in the earth's atmosphere, they get scattered in all directions. 

As the sunlight looks white but it is made up of a spectrum of seven colours which are red, orange, yellow, green, blue, indigo and violet. As Red light has the longest wavelength while blue and violet light has a much shorter wavelength. 

Due to the short wavelength of violet and blue light, it gets scattered more than other colours present in the sunlight. This is known as Rayleigh Scattering. As our eyes are more sensitive to blue light then violet, Sky looks blue in colour.