Erica's Answer:
The sun's rays hit small water droplets that linger in the air after rain. The water droplets act like a prism, and they diffuse the white light from the sun and break the light up into the different colors of the rainbow. Rainbows would not form if the sun's light was any color other than white. White light is actually a combination of all the colors in the visible spectrum, and a prism separates all the colors out to make the rainbow.- Erica
