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Nature curiosity: What causes a rainbow to appear?

A double rainbow over a prairie.
(Photo via Shutterstock)

Remember Roy G. Biv from elementary school? He's the colorful character who taught you the colors of the rainbow in order. Thanks to him (and the order of the letters in his name), you know red is at the top, then orange, yellow, green, blue, indigo and violet. 

In the same lesson in which you learned about Roy G. Biv, you probably also learned what causes rainbows to appear, but without a clever mnemonic device, that information might not be so easy to recall today. So let's go back to class for a lesson on rainbows.

Rainbows appear in the sky when just the right conditions exist. They need sunlight and certain atmospheric conditions to be visible, according to the National Oceanic Atmospheric Administration. Even then, though, you have to be in the right spot to be able to see a rainbow. That's because rainbows exist only as an optical illusion. They aren't tangible objects, so there never will be a pot of gold at the end of a rainbow.

Rainbows appear when water droplets are floating in the air and the sunlight is at our back, the NOAA reports. This is why we usually see them just after a rainstorm, when the clouds have started to move out and the sun reappears in the sky. The water droplets from the rainstorm remain in the atmosphere, and the sunlight hits them, causing a rainbow to appear. 

We see rainbows because of the refraction and reflection of light, according to National Geographic. When light hits a water droplet in the air, the light is refracted, or bent, and then reflected, or bounced back. When sunlight is reflected off the water droplet, it is separated into its different wavelengths. This is where Roy G. Biv comes in. Red is at the top of the rainbow because the red wavelength of light is the longest and it bends the least. Conversely, violet is at the bottom because the violet wavelength of light is the shortest and bends the most, the NOAA reports.

We see a rainbow as an arc, but in reality all rainbows are full circles. We only see the arc because the rest of the circle is blocked by the horizon. On rare occasions, people in an airplane may be able to see the full circle of a rainbow because they are so high above the horizon, National Geographic reports.

Have you ever been lucky enough to see a double rainbow? Double rainbows occur when the light reflected inside the water droplet is reflected a second time at a different angle, according to the NOAA. When this happens, the secondary rainbow — the one on top — displays the colors of the rainbow in reverse order: violet, indigo, blue, green, yellow, orange and red. (Go ahead and check it out on the photo at the top of this story; you know you want to.)

While we typically think of rainbows as occurring when the sun comes out just after a rainstorm, they can be seen in other conditions as well. A fogbow is a rainbow that appears in foggy conditions. Remember that rainbows need sunlight and water droplets in the air — two conditions that can be met on a foggy day as long as the fog is not too thick. Fogbows are most often seen when the fog layer is thin and the sun is shining, EarthSky reports. The colors in a fogbow are much more pale than in a rainbow because the water droplets in fog are much smaller than after a rainstorm. This is why fogbows are sometimes called white rainbows. 

Rainbows can happen at night too. This phenomenon is referred to as a moonbow or a lunar rainbow, and it occurs for the same reason as a rainbow except the light source is the moon instead of the sun, National Geographic reports. Like fogbows, moonbows are sometimes called white rainbows. The reason is different, however. Lunar rainbows are not as vivid as rainbows because the light from the moon, which is actually reflected sunlight, is much more dim than sunlight.

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