What is the reason behind rainbow formation just after rain when sunlight falls on water droplets?

A rainbow is an atmospheric phenomenon which is caused by reflection, refraction and dispersion of sunlight from condensed water droplets resulting in a "spectrum of light" (a band of colours) appearing in the sky, that takes the shape of a multicoloured arc.

Due to dispersion, white light (sunlight) separates into different components of light by their different degrees of refraction according to wavelength.

It separates into a total of seven different colours, and the band of colours starts with red on the outside and changes through orange, yellow, green, blue, indigo to violet on the inside.

It often visible after rainfall due to a highly humid atmosphere, and always appears directly opposite the sun.

Process 

When a beam of sunlight enters the water droplet, it slows down and bends towards the normal, as it goes from the rarer medium (air) to denser medium (water), and then split into separate colours.

And, upon exiting the droplet, light speeds up and bends away from the normal, as it goes from the denser medium (water) to rarer medium (air).

The angle from the ground surface to the vertex of the rainbow (visual angle), is approximately 42, while the original sun rays enter approximately at 40.