Why do rainbows appear as arcs in the sky?

A rainbow appears when sunlight enters raindrops, bends, reflects inside them, and bends again as it exits. The geometry sends the strongest light toward the observer at specific angles, about 42 degrees for red and slightly less for violet. Because all the droplets that send light to your eye lie on a cone around the antisolar point, you see an arc rather than a straight line. From an airplane, you can sometimes see a full circle. The physics of rainbows is closely tied to refraction and dispersion, explained mathematically by the 17th-century work of René Descartes and Isaac Newton.