Rainbows are formed when light from the sun refracts off of raindrops at a certain angle. That's why rainbows are often seen when the sun comes out after rain or other precipitation recently occurred. Red light is created when the angle of refraction is 40 degrees and blue (or violet) light is created at 42 degrees. All other visible light is in between the 40 to 42 degree range (a miniscule difference). All of the visible colors of the rainbow include: red, orange, yellow, green, blue, indigo, and violet.
Rainbows can also be reproduced artificially. One example of this is pure white light being shown into a prism. Prisms refract light as it enters through one side at a certain angle, bending the light. This creates the spectrum of seven colors as well.
Pink Floyd used the image of a prism and the spectrum of colors to grace the cover of his famous album "Dark Side of the Moon."
This short clip shows how a white light is altered and split into different colors with a prism.
