A rainbow is an optical phenomenon, usually appearing in the sky when a ray of sunlight refracts through millions of raindrops, sending the spectrum of component colours into the eye. For this to happen, the angle between the ray of light, the raindrop and the human eye must between 40 and 42 degrees.
Sir Isaac Newton, who first explained how it was formed and studied it in depth, was, like 14% of the human population of Earth, colour blind, so he had to rely on the eyes of his assistant, who was equipped with seven-colour-perception, and were able therefore to discern a difference between indigo and violet.
Primary Rainbows are the most common and have the most distinctive colours, with red appearing on the outside of the arc and violet on the inside.
Secondary Rainbows have less distinctive colouring and are created on rare occasions where the light ray undergoes total internal reflection twice within the raindrop before escaping. The second reflection causes the colours to blur and appear in reverse.
The most popular myth surrounding rainbows is that if you find the end of one there will be a crock of gold waiting for you. This cannot, however, be disproved, because it is impossible to find the end. The point where the rainbow appears to touch the ground recedes from you as fast as you can approach it.