Monday, June 20, 2016

Double Rainbows at Grasslands National Park

One recent evening, we were camping in Grasslands National Park when rain clouds swept in from the southwest. After they passed, two rainbows formed. The sun was setting as the clouds moved on to the northeast, and the sunset tipped the clouds with pink and lit the distant rain curtain beyond the second rainbow with a golden glow.

Double Rainbow. Copyright © Shelley Banks, all rights reserved.
But wait - the secondary rainbow has the typical order of colours reversed, with red on the inside.
As for the sunset, it was behind me when I took this picture. 

Also... Note how dark it is between the rainbows...
Sometimes the most interesting sunset effects are on the other side of the sky.  © SB

I've seen double rainbows before, but never focused closely. I thought they were similar, just one fainter than the other, I don't remember noticing that the second, outer rainbow reflects the order of colours in the opposite way. A mirror image. Backwards, with red inside the arc, not along the outside edge.

There is a great explanation of this at one of my favourite sites, Atmospheric Optics, but essentially, the second rainbow is not only reversed, but fainter and with its colours more widely spread out. And that's because of the way light is reflected within raindrops. Seriously. It's interesting. Check out the science of secondary bow formation.

As for the space between the rainbows — yes, it's darker than on the other sides of either bow. AtOptics explains that this phenomenon was first identified in 200 CE, and is called Alexander's Dark Band. I do not make this up. As I understand it, light from the inner rainbow brightens the sky inside it, while light from the outer rainbow bounces further out, brightening the outer sky, leaving that dark part in between.... It's all explained here.

What is this? A double rainbow
Location: Grasslands National Park, Saskatchewan, Canada
Photo date: June 13, 2016


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