You can also view previously answered questions in the HSW Question Archive . That is why a prism can take in white light on one side and produce its own mini-rainbow on the other side. To understand rainbows you have to start by understanding what is happening inside a prism to let it separate white light into its colors. A prism is a triangular piece of glass or plastic. To get it to produce a mini-rainbow you allow a narrow strip of white light to fall on one face of the triangle.
Every material has a different refractive index.
The angle of bending is different for different wavelengths of light. As the white light moves through the two faces of the prism, the different colors bend their different amounts and spread out into a rainbow. In a rainbow, raindrops in the air act as tiny prisms. Light enters the drop, reflects off of the side of the drop and exits, and in the process is broken into a spectum just like it is in a triangular glass prism. In a double rainbow, the second bow is produced because droplets can have two reflections internally and get the same effect.