Sunlight is the combination of light spanning the color spectrum. But when the sun’s rays shine through a raindrop, that light is forced to slow down. Each color is refracted, or bent, a different amount depending on its speed. The resulting light is separated into individual colors.
A double rainbow results when a fraction of the light exiting the raindrop instead bounces off the droplet’s inner wall. This reflection — and subsequent refraction — causes a dimmer rainbow to appear overhead, thanks to the fainter light. In the second rainbow, the order of the colors is reversed.
What happened in Maine, however, is particularly remarkable — a reflected light bow. It forms when a lake, pond or ocean reflects a projection of the sun, which generates another set of bows. Naturally, it’s not as bright, and it is most commonly seen stretching up from the primary bow. Once in a while, a fourth climbs from the secondary arc.
The formation of a reflected-light bow requires a mirror-like surface, meaning the water has to be flat-calm. Getting those conditions on the ocean is extremely challenging, but the protective inlet of Passamaquoddy Bay off the Bay of Fundy served as shelter from rough seas.
Reflected-light bows open up wide, fanning out as they tower upward. Because they’re a reflection of the primary and secondary bows, the imaginary “center” of the circle is above the horizon — as opposed to below for traditional bows. That’s because the sunlight for a reflected light bow appears to come from below, rather than above. It’s like when sunlight shines upward from a puddle of water and temporarily blinds you.
Despite the rarity, this is not what a triple or quadruple bow would look like. Tertiary and quadruple rainbows are real things, first photographed in 2011. Third- and fourth-order rainbows are unlike their lesser-order siblings, instead appearing on the same side of the sky as and centered on the sun. The rain has to be opposite the sun for the first two bows, and then any rain to the left and right of the sun is the canvas for the third- and fourth-order bows — but at the same time, the sun has to be unobstructed. It must also shine brilliantly enough that, amid all the reflections within the raindrop, enough light is left over for that fourth arc to shimmer. Lining up those conditions is about as tough as winning the lottery.
Now how about that pot of gold?