Why are rainbows curved?

Rainbows are curved because of the way sunlight interacts with water droplets in the atmosphere. When sunlight enters a spherical raindrop, it bends—a process called refraction—as it passes from air into water. Inside the droplet, the light reflects off the inner surface and bends again as it exits. This combination of refraction and internal reflection causes the light to spread into its component colors and exit the droplet at a specific angle relative to the original path—typically about 42° for red light and slightly less for blue.

This process occurs simultaneously in countless droplets across a rain-filled sky. However, only those droplets positioned at the correct angle with respect to the observer’s line of sight reflect light that reaches the eye. These droplets lie on the surface of an imaginary cone, with the observer’s eye at the tip and the cone pointing away from the Sun. The visible portion of a rainbow from the ground is an arc—a segment of a circle—while the lower half is typically obscured by the horizon. From a high vantage point—such as an airplane—observers can sometimes see the entire circular rainbow, provided the angle between sunlight and water droplets is just right.