How do double rainbows form?
Residents of Palmer caught a spectacular show late Monday after a round of light showers swept through the area. The perfect atmospheric conditions meant some saw a beautiful double rainbow.
The right ingredients
Rainbows need just the right conditions to form; even a single rainbow spread across the sky is quite complex.
For starters, you need sun behind you and rain in front of you. Second, the sun must be relatively low on the horizon — 42 degrees or lower is the magical angle.
With those requirements met, the science begins.
Reflection and refraction
The reason those ingredients are so important is that they need to be present for a spectacular bit of science to occur.
The light that is emitted from the sun is white, or the entire spectrum of the rainbow mashed together. By containing all the colors of the rainbow, sunlight allows us to see the colors in the world around us.
When that white light hits a raindrop, something interesting occurs. The light is bent, or refracted inside the drop of water. The refraction is what divides white light into the light spectrum. This means that as the light enters, the raindrop it is divided.
The divided light then hits the other side of the raindrop and reflects. That sends it back toward the sun and the observer. As it reflects, the spectrum broadens, further separating out the colors of the rainbow.
As the light finally exits the raindrop, it refracts one last time. The final product of all of this reflection and refraction is what we see, a rainbow!
On occasion, the right conditions can produce a double rainbow. The secondary or fainter rainbow occurs on top of the original rainbow and the mechanism that produces the second rainbow isn’t much more complicated than the rainbow itself.
The second rainbow looks like a reflection of the first. The color on top of the brighter rainbow is the color on the bottom of the fainter rainbow. The reason the second rainbow looks like a reflection of the first is because, in a way, it is.
The second rainbow comes from the same scientific process that creates the appearance of the first, there is just one more reflection inside the raindrop. That second reflection is what makes the rainbow appear upside down compared to the original and also what makes it appear higher in the sky.
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