The sky is blue because of:

The reason the sky is blue is due to dust particles scattering light.
When sunlight enters the atmosphere of Earth, all the gases and airborne particles disperse it in all directions.
Because blue light travels as shorter, smaller waves than the other colours, it scatters more than the other colours.
This explains why the sky is often blue.Light reflection: Some surfaces have the ability to bounce light back into the original medium when it touches them.
The reflection of light is the process through which a surface sends light back into the same medium.
The reflection law:
The angle of incidence and the angle of reflection are the same.
They all lie in the same plane: the incident ray, the normal to the mirror at the point of incidence, and the reflected ray.Refraction of light: Refraction of light is the term used to describe how a light beam bends as it travels through different media.
Refraction laws:
At the point of incidence, the incident ray, the refracted ray, and the normal to the interface of two transparent media all lie in the same plane.
Snell's law states that there is a constant relationship between the sines of the angles of incidence and refraction.
Dispersion:
Figure: white light's diffraction through the glass prism
the process of separating visible light into its many colours.

Sky is formed due to dispersion and scattering of light. When a parallel beam of light passes through any medium full of molecules and any other particles light gets scattered in all possible direction, and thus blue scatters more as it has smaller wavelength.