Bunge’s photo quickly went viral, capturing the stunning double rainbow as it arched over the U.S. Capitol. Shafts of rain can be seen plummeting from angry skies overhead, the darkness above in stark contrast to the brightly sunlit and amber-washed trees below.
Mukul Ranjan, a D.C.-based photographer, snapped a similarly striking photo two miles west from the steps of the Lincoln Memorial, showing the brightly illuminated Washington Monument standing proud and defiant against stormy skies. A double rainbow spans the sky, perfectly framed and centered above the obelisk.
The environment Sunday was primed for spectacular rainbows across the D.C. area, with an assortment of meteorological factors combining with pure luck in some cases. Even the rainbows that formed were extra special, packing additional bands of colors into each vibrant arch.
What makes a rainbow?
Rainbows form when sunlight enters a raindrop and its speed of propagation slows down. Sunlight contains every color of the visible spectrum, but all colors shine at once, meaning we usually only see white light. But when that light enters a raindrop, each component color slows and is bent, or refracted, at slightly different angles, reflecting off the back edge of the raindrop before exiting back toward an observer. Since the colors no longer overlap, each shade can be seen individually.
Violet light, which has the shortest wavelength, is refracted the most, and appears on the inside of the primary arc. Red, with a much longer wavelength, lines the outside of the rainbow.
Sometimes, a double rainbow forms — the result of some of the sunlight reflecting, or bouncing off the inside of a raindrop a second time. That second bounce flips the colors. Since most of the sunlight exits the raindrop after one bounce, there usually isn’t much left behind to form a secondary bow. That’s why it’s generally fainter.
Sunday’s bows contained extra colors
Observers in and around D.C. noticed some additional colors inside the primary arc during Sunday’s show, too — with up to three repeating bands of colors packing the inner rim of the rainbow. Sunday’s rainbow was a “supernumerary” bow, and formed thanks to a high number of smaller raindrops present in the atmosphere.
When two beams of sunlight enter a raindrop, even though they’re parallel when they enter, they strike the back edge of the raindrop in slightly different places, and subsequently are bounced in acutely different directions when they exit. For an individual color, which is composed of a wavelength of alternating crests and troughs, that means the two light beams may no longer overlap “in phase.”
The out-of-sync waves of light then interfere with one another — crests aligning with crests and troughs with troughs in “constructive interference,” and the opposite occurring with “destructive interference” — and produce alternating strips of light and darkness as a result.
The central pocket of constructive interference produces each bright band of color we see in the primary rainbow; then each iteration of a supernumerary band is the product of subsequent instances of constructive interference, which grow dimmer each time before fading away.
Smaller raindrops will make for more widely spaced supernumeraries. Supernumerary bows are best when a storm produces raindrops roughly uniform in size.
Other ingredients for a stunning show
Both Bunge’s and Ranjan’s photos also reveal Alexander’s Dark Band, or the relative minimum in light between a primary rainbow and its accompanying second bow. Sunlight reflected by raindrops in that region can’t reach an observer because of the angles it takes upon exiting the raindrop, leaving an arc of darkness. The band occupies 8 degrees of the sky between the primary and secondary bow.
A contributing factor to the splendor that graced Sunday’s skies was the isolated nature of the storms. The formation of pop-up showers and downpours meant it was easier for sunlight to peek through the gaps behind individual storm cells.
A drying westerly wind and low humidity made for higher cloud bases, allowing the rain to pour down from greater heights and form the perfect backdrop for beautiful bows.
Below, find some more magnificent rainbow scenes from our Twitter followers.
Jason Samenow contributed to this article.