What causes the vivid colors in a rainbow, and why is it always in a perfect arc?
Raindrops are responsible for the colors of rainbows. They break the sunlight up into the full spectrum of colors, each drop acting as a sort of miniature prism. Each color emerges at a slightly different angle on each raindrop. When millions of raindrops gather, the spectrum shows up as distinct bands of color — each with a different length. Red is the longest and violet the shortest band. The arc is most pronounced when the Sun is close to the horizon.