Rainbows are caused by the refraction, dispersion, and reflection of sunlight as it passes through raindrops in the atmosphere. When sunlight enters a raindrop, it slows down and bends (refraction) because light travels slower in water than in air. This bending causes the light to spread out into its component colors (dispersion) because different colors bend at slightly different angles. The light then reflects off the inside surface of the raindrop and is refracted again as it exits the droplet, directing the separated colors toward the observer's eye. The observer must be positioned with the sun behind them and rain or water droplets in front for a rainbow to be visible. The colors appear in a circular arc, with red on the outer edge and violet on the inner edge. A full circular rainbow is possible but usually only a half arc is seen from the ground due to the horizon blocking the rest.
