Science - Physics and forces - Rainbow optics
Why does a rainbow have an arc shape?
Correct!
Sunlight is refracted as it enters raindrops, reflected inside them, and refracted again as it leaves. The different wavelengths of white light separate into colors. Because the drops are round and light reaches the eye from particular angles, the geometry produces a complete circle. From the ground, the horizon and Earth usually hide the lower part, so we normally see only an arc. From an airplane, the full circle can be visible under suitable conditions.
Source: National Weather Service
