A rainbow is round because the process is based on angles. Light from the sun hits the little water droplets after a rain. The water droplets act like little prisms. Different colors are sent out at different angles. The sun must be behind you. Imagine a narrow triangle. The narrow angle is at the water drop. One side points toward the sun. One side points toward your eye. The drops for which this device works form a circle. The center of this circle lines up with you and the sun. If the Earth were not in the way, a rainbow would be a complete circle. This is why you can never find "the golden pot at the end of the rainbow". A rainbow is really a complete circle.