Water drops from rainfall or sprinklers create natural prisms that reflect the light from the sun. When the rays of the sun only bounce once off the inside of a water drop, the rainbow is much brighter. The more the sunlight bounces off the inside of a water drop, the dimmer the rainbow becomes. The sun enters the falling water droplet and reflects back to the human eye from the opposite side of the water drop.
The white light from the sun bends on the angular surface of the water drop. Once the bending of the sun begins, the white light breaks up into colors. Three different things must occur at the same time to allow the sun to perform this display of beauty. The sun must be behind you, the sun has to shine and water drops must be in front of you. Light from the sun is actually bent twice to form the rainbow. It is bent once as it enters the water drop, then bends again when the colors exit the water drop.
The human eye can see as many as seven colors in a rainbow. Red, yellow, orange, violet or purple, green, blue and indigo or dark blue are the colors that people can see, but sunlight is made from numerous colors. When all the colors of the sun combine, they make up the color white. One water drop cannot make a rainbow. In fact, each water drop only reflects one color of the rainbow. Several water drops must be present to see all the colors of the rainbow.
A double rainbow is created by the reflection of sunlight bouncing off the water drop twice. The second bounce or reflection exits the water drop at a different angle than the first reflection. The first bounce of sunlight off the water drop is called the primary rainbow and the second, dimmer rainbow is called the secondary rainbow.