Why does a rainbow always curve like a bow?

Sunlight enters a raindrop, reflects once inside, and bends as it leaves. Most of this light gathers in directions about 42 degrees from the point directly opposite the sun—the point your head's shadow indicates. Connecting all positions 42 degrees from that point makes a circle, and the droplets along it glow as a rainbow. Each color bends slightly differently, placing red on the outside and violet on the inside. Sometimes a fainter rainbow appears outside the first. It is made by light that reflects twice inside the raindrops, at about 51 degrees, and its colors are in reverse order. The sky between the two bows looks unusually dark because less light reaches you from there.

Because the circle's center is opposite the sun, it lies below the horizon while the sun is up. Around sunrise or sunset, the center is on the horizon and the rainbow looks almost like a semicircle. As the sun rises higher, the arc becomes lower and smaller. Above 42 degrees, a rainbow cannot be seen from the ground. This is also why we encounter rainbows more often in the late afternoon than at midday. For the same reason, rainbows appear in the eastern sky in the afternoon and in the western sky in the morning. After a shower passes, try standing with your back to the sun.

It may seem that you could reach the end of a rainbow by heading toward it, but as you walk, the rainbow moves back just as far. A rainbow is not an object hanging in one place. It is light defined by the angle between the sun and your eyes. When you move, the raindrops sitting at the 42-degree position change too, so you and the person next to you are actually seeing rainbows made by different raindrops. This is also why a rainbow seems to follow you when you ride in a car.

The bottom of the circle points toward the ground, where there are few droplets between your eyes and the surface to return the light. If droplets fill the space below you too, more of the ring can appear. For example, on a sunny day, spraying a broad mist with a garden hose while facing away from the sun can sometimes create a rainbow curving all the way toward the ground.