Why Is a Rainbow Really a Circle?

A rainbow usually appears as an arch because the ground hides the rest of it. Its complete shape is a circle centered on a point directly opposite the Sun.

Sunlight enters millions of raindrops, bends, reflects from the rear surface, and bends again as it leaves. The strongest returned light reaches an observer at a particular angle, about 42 degrees for the red edge of a primary rainbow.

Every droplet located at that angle from the point opposite the Sun can contribute light. Together, those possible droplets form a circle around the observer's line of sight.

From the ground, droplets below the horizon are usually blocked by the landscape, leaving only the upper arc visible. The lower the Sun sits behind the observer, the more of the rainbow rises above the horizon. When the Sun is too high, the circle may fall almost entirely below the visible landscape.

From an airplane, mountain, or high waterfall, enough illuminated droplets may surround the observer to reveal nearly the entire circle.

Every person sees a slightly different rainbow because the necessary angles are measured from that person's eyes. Even two people standing together receive light from different droplets. As the observer moves, the contributing droplets change as well.

A rainbow is not a colored object hanging at one location. It is a personal circle created by sunlight, water, and viewpoint.