Can You See A Rainbow After Sunset? Absolutely

Tuesday evening’s rainbow was very red because it happened just before sunset. Air scatters away the blues and greens from the low sun, so only warmer colors enter the raindrops and are reflected back. You can also faintly see part of the second bow (at right) and faint supernumBob King

I took a long nap the other evening. And when I awoke, the world had changed. The sky, the air, everything glowed yellow-orange. Still drunk on sleep’s melatonin I stumbled outside and took a walk down the road. It started to rain about the time I became fully conscious. Instinctively, I looked back over my shoulder and sure enough, there appeared a rainbow.

This is a normal double rainbow with a full complement of colors. Bob King

The sun was very low at in the northwestern sky and lighting up rainclouds in fantastic pastels. The rainbow wasn’t especially bright but intense enough to see the second fainter bow outside the primary. Sunlight entering a raindrop is refracted or spread out into the spectral colors exactly as if the droplet were a prism. Each color of light reflects off the back of the droplet and out again. The transparent inner wall of a raindrop allows some of the light to pass, while reflecting a portion back — exactly the way a window allows light to pass while also faintly showing your reflection when you look into it.

As the light exits, it’s refracted a second time in the direction of your gaze. Now, it’s even more spread out or dispersed. All the angles involved in refraction and reflection deliver the rainbow light to your eye only when the sun is low in the sky.

Sunlight enters a raindrop, reflects off the back surface and shoots out the other side. Bent twice, light spread out into its component colors. UW-Stout

A single in-and-out reflection inside a raindrop (times a trillion raindrops) creates the bright and colorful primary bow. The secondary bow results from light that reflects twice inside a raindrop before exiting. Since light is lost through the raindrop on each reflection, the secondary is fainter than the primary. It also shows the usual color sequence — red, orange, yellow, green, blue, indigo and violet — in reverse. 

The secondary bow forms from a double reflection inside the raindrop. Image from the University of Illinois WW2010 Project

It was now only minutes before sunset, and the rainbow was quickly growing fainter and redder. I could still see some green and purple, but the overall impression was red. That’s easy to understand. Near sunset and sunrise, the atmosphere scatters away (removes) the cooler blue and green hues from white light, making the sun appear red or orange. With only warm colors from the setting sun enter a raindrop, only warm colors emerge.

Here’s the last stub of red rainbow in the eastern sky 7½ minutes after sunset. Bob King

What I found amazing was how long I was able to keep parts of the rainbow in view after the sun had set. Sundown on June 12 occurred at 9:03 p.m., yet I kept a “leg” of the bow in view until 9:11 p.m., fully 8 minutes after sunset! At 9:11 p.m., the sun stood exactly 1.2° below the northwestern horizon.  I had to assume that raindrops in the distance were at a high enough altitude to still be touched by sunlight.

Part of the sunset rainbow arcs over a lake in this magical sunset scene photographed on August 21, 2016. Andy Sajor

But it got me to wondering if the strength of twilight alone might have been enough to fire up a last-gasp rainbow.

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