How can rainbow form?

In order to answer the science facts question: How can rainbow form?, let's have a look the article as below:

One of nature's most splendid masterpieces is the rainbow. A rainbow is an excellent demonstration of the dispersion of light and one more piece of evidence that visible light is composed of a spectrum of wavelengths, each associated with a distinct color. To view a rainbow, your back must be to the sun as you look at an approximately 40 degree angle above the ground into a region of the atmosphere with suspended droplets of water or even a light mist. Each individual droplet of water acts as a tiny prism that both disperses the light and reflects it back to your eye. As you sight into the sky, wavelengths of light associated with a specific color arrive at your eye from the collection of droplets. The net effect of the vast array of droplets is that a circular arc of ROYGBIV is seen across the sky. But just exactly how do the droplets of water disperse and reflect the light? And why does the pattern always appear as ROYGBIV from top to bottom? These are the questions that we will seek to understand on this page of The Physics Classroom Tutorial. To understand these questions, we will need to draw upon our understanding of refraction, internal reflection and dispersion.

The birth of each rainbow begins with millions of tiny rain droplets. The rain droplets serve as a type of reflector of light. White light enters one individual rain droplet and exits as one specific color of the spectrum. Without millions of rain droplets, a rainbow would not occur. If you only had a few rain droplets you would only see a few colors. This is typically why rainbow appear after a rain storm.

Each rain droplet has a function in the formation of the rainbow. Sunlight enters the rain droplet at a specific angle and the rain droplet separates the white light into many different colors. 

This angle is a fixed measurement between your eye and the sun. What color is refracted depends upon the critical angle, which is the angle the sunlight strikes the back of the rain droplet. Red light bends the least, exiting the rain droplet at a 42 degree angle, while Violet light bends the most, exiting the rain droplet at a 40 degree angle. All of the other colors of the rainbow exit the rain droplets at some angle between 40 and 42 degrees, thus making up the colors of the rainbow ROYGBIV, this order never changes.

Each rain droplet reflects all colors at a given point and time, but only one color comes back to your eye, requiring million of rain droplets to create a rainbow. As the rain droplets fall through the sky, the colors of the spectrum being reflected and refracted are constantly changing.

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