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Does the back of a rainbow look the same as its front side?
Category: Earth Science
Published: July 3, 2016
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
A rainbow does not have a back side. If you were to walk completely to the other side of the mist cloud that is creating the rainbow and turn around, you would not see a rainbow. You have to realize that a rainbow is not a stationary physical object. Instead, it is a pattern of light that becomes a stable image only when you look at it from the right angle. You may not have noticed it, but every time you look directly at the center of a rainbow, the sun is directly behind your head. This is the only angle at which the light pattern that constitutes the rainbow can enter your eye and therefore lead you to see it.
The sun is always in the opposite part of the sky from the center of the rainbow. This is because a rainbow is actually just sunlight which has been refracted and reflected. Refraction occurs when the sunlight enters and leaves the small spherical water droplets that constitute the mist. This refraction is what causes the rainbow's spread of colors and arching shape. However, the overall location of the rainbow is determined mostly by the step in the process where the sunlight is reflected off of the inner back surface of the water droplets. The sunlight that reflects only once off the inner back side of the mist droplets is what constitutes the primary rainbow. Additionally, a small amount of the sunlight reflects twice off the inner back surface. This light consequently comes out of the mist droplet at a slightly different angle, leading to the secondary bow. The secondary bow is always there, but it is so dim that humans can only see it during clear viewing conditions. This second reflection only changes the light's direction a small amount. As a result, both the primary bow and the secondary bow can only be seen when looking away from the sun. Furthermore, neither one can be seen from the back side, i.e. when looking toward the sun.
This point brings up other interesting questions. Can't some of the sunlight pass through the back side of the water droplet without being reflected? Wouldn't this light be visible from behind the rainbow? The answers are yes and yes. Although it is only a small amount, some of the sunlight that enters a mist droplet indeed continues through the back side without being reflected. Therefore, if you were to walk to the other side of the cloud of mist that is creating the rainbow and turn around, you would indeed see a pattern of light (if the viewing conditions are favorable). However, it would not be a rainbow. It would be a pattern called an "atmospheric solar corona", as shown below. A solar corona still includes an arcing shape and a spread of colors, but the size and color sequence of a solar corona are different that of a rainbow. Since most of the sunlight that enters the mist droplet is reflected and not transmitted through, rainbows are very bright and common while solar coronas are dim and rare. Additionally, since you are behind the rainbow, you are now looking directly toward the sun. The sun's glare therefore makes it hard to see the solar corona.
