Double Rainbows

In 2014, I had the phenomenal opportunity to travel to Yellowstone National Park. As part of our hiking tour, we visited Uncle Tom’s Trail which consists of over 300 steel steps built down the south wall of the Grand Canyon of the Yellowstone River next to the park’s most impressive waterfall, the Lower Falls.  It was a beautiful sunny day, and we happened to be hiking down the stairs close to noon, when the sun was almost directly over head. As I descended the staircase, the incoming sunlight hit the mist of the waterfall at the perfect angle to produce a brilliant rainbow (not something you normally get to see from above).

A rainbow forms when light enters water droplets, refracts (bends), reflects of the inner wall of the water droplet, and then refracts again as it leaves the water, returning to air and travelling to your eye. Light of different wavelengths refract slightly differently, causing them to leave the water droplet at slightly different angles. This means that light of one wavelength (violet) would make it to your eye from a water droplet that is high in the sky, where another wavelength (red) would require a lower water droplet to allow the light to reach your eye. A rainstorm (or in this case waterfall) provides billions of water droplets, which produced an accumulated effect of sending various colors to your eyes from different positions in the sky. No two observers will see exactly the same rainbow, because the angle of the light to their eye is slightly different.

On occasion, you might find yourself at the proper angle that allows light to reach your eyes after reflecting twice within the water droplets. This produces a secondary rainbow, which is inverted in color. As I climbed down the staircase, I noted that by going deeper into the canyon, the rainbow got brighter. A few steps further, and a secondary rainbow appeared! While the math would show that only a small change in viewing angle is necessary to view this phenomenon, I was elated to watch the secondary rainbow appear, and then disappear when I continued just a few more steps down the staircase.

If you are interested in learning more about rare and unusual rainbows, I encourage you to take a look at Professor Mike Merrifield’s Sixty Symbols video about circumzenithal and supralateral arcs.

In his video, Professor Merrifield references the Astronomy Picture of the Day. If you are not familiar with it