Rainbow is a circular arc of colors that appears in the sky when raindrops are illuminated by sunlight. A rainbow is not a physical object. Rather, it is a pattern of light to which a great number of raindrops contribute. A rainbow may spread across the entire sky, and its ends may seem to rest on the earth. Not all rainbows form complete arcs, however, because a rainbow cannot appear in a part of the sky where there is no rain.