What causes a rainbow?

A rainbow forms when sunlight interacts with water droplets in the atmosphere. As light passes through a droplet, it refracts, bending at an angle, and then reflects off the back of the droplet before refracting again as it exits. This process separates white light into its component colours, creating the familiar arc. The position of the observer relative to the sun and the water droplets determines the visibility of the rainbow. For a full-circle rainbow, the observer must be positioned directly between the sun and the water source, such as from an aeroplane or a mountain peak. Most rainbows appear as arcs because the ground blocks the lower portion of the circle.