One of the most majestic weather phenomena is the rainbow. Although it’s not rare to see a rainbow, what is rare is the occurrence of a supernumerary rainbow. These rainbows take on physics that are different of a primary rainbow (one that we are all familiar with) and that of a double rainbow.
A rainbow is formedas light passes through a water droplet. The droplet behaves much like a prism. As the light enters the water droplet, it is refracted (bent). The part that is refracted back to you is the rainbow you see and to see the rainbow, you have to be opposite the sun.The angle that is refracted gives the colors of the rainbow that our eyes see. The colors of the rainbow are red, orange, yellow, green, blue, indigo, and violet (ROYGBIV) and are always in that order. To see the red color of the rainbow, light needs to be refracted at an angle of 42 degrees into our eyes. The blue portion, on the other hand, refracts at an angle of 40 degrees. Red will always be at the highest angle and violet at the lowest; refer to ROYGBIV. The image directly below shows how light is refracted at a certain angle when the light enters and leaves a water droplet. The second image below graphically depicts which colors you are able to see with respect to the refracted angle of the sun’s rays. Notice that the colors follow the ROYGIV pattern from top to bottom.
Image Courtesy: Met Office
Image Courtesy: Met Office
Unlike the primary rainbow, supernumeraries can only be explained from the wave nature of light, not from geometric optics.Generally speaking, supernumeraries are miniature arcs of rainbows which reside just inside the primary rainbow and mimic the primary rainbow in color and shape.Constructive and destructive interference are the main principles in forming supernumerary rainbows. The interference occurs between rays of light following slightly different paths with slightly varying lengths within the raindrops. Constructive interference occurs when the light waves are in phase with each other. This enables a bright band to form. When the waves are out of phase with one another, this is known as destructive interference, resulting in a gap. Destructive interference results in darkness, while with constructive interference, there is light. These rainbows are also most visible when the raindrops are of similar size. If there is too much variation in raindrop size, some of the colors within the supernumeraries will be washed out. The most vibrant colors in this case will be red and green. Below is a photograph of a supernumerary rainbow. Notice how the secondary rainbows are a mirror image of the primary rainbow, just fainter and smaller.
Credit: NASA, Met Office
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©2019 Meteorologist Corey Clay