Supernumerary bows are alternating green-pink-green bands appearing inside the primary rainbow. Unlike the main rainbow (explained by refraction and reflection), supernumeraries are explained by wave interference: refracted rays from same-sized drops interfere constructively and destructively.
They are most visible when raindrops are uniform and small (0.5–1 mm). With large or mixed-size drops, interference patterns blur and are not observed. They are a direct demonstration of light's wave nature.