Supernumary Arcs

If rain droplets are uniform and of the right size, ~0.2 mm in radius, weak blue to violet bows, called supernumary arcs, appear inside the primary bow. As many as six arcs are sometimes seen.

Sunlight is partially coherent over short distances, (typically over 0.1 mm). The incident and reflected beams overlap inside the rain drops, producing interference that leads to the effect..The arcs were of importance historically, because Thomas Young was able to explain their formation on the basis of his wave theory of light.

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