The Raindrop Size Rule Behind Supernumerary Rainbows

A supernumerary rainbow isn't a trick of the eye so much as a trick of uniformity. It only forms when the falling water droplets are all nearly the same size and typically less than a millimeter across. When that condition holds, sunlight doesn't merely reflect from inside the raindrops — it interferes, the same wave phenomenon you get as ripples spreading on a pond after a stone is thrown in. The fainter extra arcs are an interference pattern, which is why they depend on a narrow droplet-size distribution rather than on the storm simply being large.