The Elusive Fifth Order Rainbow Has Been Photographed for the First Time

Until recently, no one was sure anything more than a double rainbow could ever occur in nature.

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Oct 21 2014, 12:00pm

Unedited photo taken with polarized lens. Image: Harald Edens

Harald Edens, a Dutch-born atmospheric scientist, carries out his research on the top of a mountain, in the middle of rainstorms. The Langmuir Laboratory for Atmospheric Research sits at 10,000 feet in the Magdalena Mountains of New Mexico, where Edens studies lightning and thunderstorms. Over the past 14 years, he's taken up another hobby: running out at the tail end of a storm to photograph rainbows in the hope of catching a glimpse of a fifth order rainbow, which scientists weren't even sure could be seen in nature.

It's a phenomenon that's only barely visible to the naked eye, and Edens had never seen one. But he was confident his camera could find it. On August 8, 2012, at the end of a weakening thunderstorm, he snapped 100 pictures of a double rainbow in 13 minutes. He used his Nikon DSLR and a polarized lens to make the colors clearer. When he pulled the photos up on his computer later, Edens did a simple enhancement of adjusting the color contrast, and suddenly the fifth order rainbow was illuminated.

"If you look closely, you can see the green hue where the fifth order bow is," Edens said in an interview."That untouched photo is the same you'd see with your eye if you were looking through polarized filter." With sharp eyes and some helpful photo editing software, Edens was able to capture it. He published his findings in the journal Applied Optics.

fifth order rainbow EDITED.jpg

Edited photo with increased contrast. The arrow points to the beginning of the fifth order rainbow. Image: Harald Edens

Rainbows form when light bounces off the inside of water droplets; the "order" of a rainbow indicates how many times the light has bounced. So a fifth order rainbow forms only when light is able to bounce inside a droplet five times. In the lab, reflection within a water droplet has been observed occurring up to 11 times, so scientists know that higher order rainbows are possible. But in nature, conditions are rarely so perfect, because rogue droplets of water vapor or other impurities in the air scatter the light.

Although it may seem a strange location, the Langmuir Lab actually boasts impeccable conditions for crystal-clear natural rainbows. The air is free of most impurities, and, at 10,000 feet, it is only two-thirds as dense as air at sea level. There's less air to scatter the light.

Until recently, no one was sure anything more than a double rainbow could ever occur in nature.

"The third order had only been observed a couple of times in the last 100 years," Edens said, and no one had ever captured it on film. Falling raindrops, especially large ones, flatten out on the bottom because of air resistance, so more than one or two reflections of light within a perfectly round drop seemed unlikely if not impossible.

But in the past three years, the scientific consensus has shifted. When Edens presented his photographs of third and fourth order rainbows at a conference in 2013, "it really became clear that these higher-order bows are really not rare," he said.

It's a bit embarrassing, I never thought I would be the one to discover this

Edens said that the advent of high-quality digital cameras has made it much easier to detect higher order rainbows.

"With digital photography you can adjust contrast and brightness of photos on a computer," he said. "That makes it much easier to bring out the color contrast and detail in the photo that otherwise wouldn't be seen."

Indeed, these higher-order rainbows became observable to someone who just looked for one season, Edens said. In 2011, a German atmospheric scientist went out looking for third- and fourth-order rainbows, and he found them fairly quickly by stacking digital photos of the same scene to increase contrast. This jogged Edens' memory, too; he had already taken many photos looking for fifth order rainbows in the early 2000s, he said, but he "pretty much forgot about it" until the announcements in 2011. Then Edens' hunt was back on. "People at the university weren't quite laughing at me, but they were like, 'oh yeah, this guy,'" he said.

Edens admits that this discovery doesn't have much practical application, but it helps the scientists in his field better understand the physics and atmospheric effects of rain droplets.

Edens is honored and surprised to have been the first to catch a glimpse of the fifth order rainbow. "It's a bit embarrassing, I never thought I would be the one to discover this," he said. "Everybody knows the rainbow, but people find it interesting nonetheless."

He hopes that his discovery will inspire more people to look for higher order rainbows. The sixth and seventh orders will be much harder to see, Edens said. "But you never know."