We see rainbows because of an optical phenomenon that occurs when sunlight interacts with water droplets in the atmosphere. Sunlight, which appears white but is actually made up of many colors, enters a water droplet and bends (refracts) because light travels slower in water than in air. Then, the light reflects off the inside surface of the droplet and bends again as it exits. Different colors of light bend by different amounts, with violet bending the most and red the least. This separation of colors creates the spectrum of colors visible in a rainbow. The sunlight must be behind the observer, and water droplets must be in front for the rainbow to be seen, typically just after rain when such conditions are met. The rainbow is generally seen as an arc because the ground blocks the full circle of the rainbow, which can sometimes be seen in full from an elevated position like an airplane.
