What’s up with these rainbows? The peculiar triplet of bows appeared to Dave Roberts (Member 54,479) one afternoon over Jelsa, Rogaland, Norway. The thin, faint innermost one is known as a supernumerary bow. It is an occasional, though not rare, effect formed by the light waves emerging from small and consistently sized raindrops interfering with each other to form fine fringes of colour that appear on the inner edge of a primary rainbow. The primary bow here is the inner of the two main bows.

More unusual by far is the outer bow that Dave spotted. This is not the common secondary rainbow, whose arrangement of colours is the other way around and whose arc is concentric to the primary bow. No, Dave’s bow diverges from the primary rainbow at its upper end. To identify it, you need to know what was directly behind him as he spotted it.

Off to the west, where the Sun was lowering in the sky, was a large body of water called Boknafjord. The rare outer rainbow was a reflection bow, produced by sunlight illuminating the rain shower after first reflecting up from the surface of the calm fjord behind Dave. Since the reflected rays hit the raindrops from a different angle to the direct sunlight, the reflection bow was centred at a different point from that of the primary rainbow. It appeared therefore at a different angle to its primary-bow relative. ‘What treasure,’ wondered Dave, ‘might one find at the foot of such an abundance of rainbows?’