What Causes a Double Rainbow?

A double rainbow is a rare meteorological phenomenon that occurs when light is refracted, or bent, twice as it passes through water droplets in the atmosphere. It is a result of the combination of specific conditions in the atmosphere and the unique properties of light.

To understand what causes a double rainbow, it's helpful to first understand the process of how a single rainbow is formed. When light passes through a droplet of water, it is refracted, or bent, as it enters the droplet due to the difference in the refractive indices of air and water. This causes the light to be separated into its different colors, with the shorter wavelengths (such as violet and blue) being refracted more than the longer wavelengths (such as red and orange).

As the light exits the droplet, it is again refracted due to the change in refractive index. This results in the formation of a circular rainbow, with the colors appearing in the order of the rainbow (red, orange, yellow, green, blue, indigo, and violet).

A double rainbow is formed when there is an additional reflection of light inside the water droplet, causing the light to be refracted a second time as it exits the droplet. This second refraction results in the formation of a second, fainter rainbow above the primary rainbow. The colors in the second rainbow appear in the opposite order of the primary rainbow, with violet on the outside and red on the inside.

Double rainbows are relatively rare, as they require specific conditions in the atmosphere for the light to be refracted twice. These conditions include the presence of water droplets in the atmosphere, the right angle of the sun's rays to the water droplets, and the right angle of the observer to the sun and the water droplets. When these conditions are met, the result is a beautiful and awe-inspiring sight that is sure to leave a lasting impression.