Double rainbow and lightning: Fantastic photo by Joan Wallner.

# Picture of the Week: Double Rainbow and Lightning

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May 7 2014 7:30 AM

# Double Rainbow, What DoeszZZZZZZZZTTT!

This picture is brought to you by Being at the Right Place at the Right Time With the Right Equipment.

Yes, that’s a double rainbow with lightning inside of it, because why wouldn’t it be?

The photo was taken by Joan Wallner in July 2010. She was driving home from Yellowstone National Park in the Badlands in South Dakota when she saw a storm starting to form. She took some pictures after the storm, which passed as the Sun was setting. Facing east (away from the Sun), the double rainbow appeared, and the dwindling storm decided it would provide the flash for her.

Photographs of rainbows and lightning together aren’t terribly uncommon, but this is one of the best ones I’ve seen. Rainbows form when sunlight passes through raindrops. The light gets bent entering the drop, reflected off the back side, and then bent again as it leaves the front of the drop. The reflection means the Sun has to be behind you to see the rainbow, and the bending separates out the colors (different wavelengths of light bend different amounts).

The most common angle for light to be bent is around 138° (where 180° would mean a perfect reflection of light, so it travels directly back in the direction from whence it came). 180 – 138 = 42, so a rainbow is about 42° in radius. Another way to put it: If you put the Sun directly behind you, so you are facing 180° away from it, the rainbow will form a circular arc 42° from that point.

However, there are other angles the light can come out of the raindrops. If it gets reflected twice inside a drop, it can come out at an angle of 129°, forming a 51° rainbow, outside the primary. The fainter secondary rainbow has the colors reversed (because the light’s reflected twice inside the drops; it’s a reflection of a reflection and the color order reverses). You can see that in Wallner’s photo.

Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!