Rainbows form when sunlight interacts with water droplets in the atmosphere through a process involving refraction, reflection, and dispersion of light. White sunlight, which is made up of a spectrum of colors, enters a raindrop and bends (refraction) because light changes speed when it passes from air to the denser water. Inside the droplet, the light reflects off the back surface. As it exits the droplet, it bends again. Different colors (wavelengths) of light bend by different amounts, with red bending the least and violet the most. This separation of colors creates the spectrum seen in a rainbow. For an observer to see the rainbow, the sun must be behind them, and water droplets must be in front of them, typically appearing after rain when the sky is partly clear. The rainbow appears as an arc in the sky, and it is a circle if viewed from an elevated position like an airplane. This explanation involves the key optics principles of light refraction, internal reflection, and dispersion by water droplets.
