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Conditions Must Be Just Right for Rainbows to Form

Have you met Roy G. Biv in school? He's the colorful character teachers often use to teach kids the colors of the rainbow in order. Thanks to him and his unique name, it’s easy to remember red is at the top of the rainbow, then orange, yellow, green, blue, indigo and violet. 

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

If you learned about Roy G. Biv, you might also have learned what causes rainbows to appear. There isn’t a clever trick to remembering that, though, so let’s brush up on it here. 

Rainbows appear in the sky when just the right conditions exist. They need sunlight and certain atmospheric conditions to be visible. Even then, 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.


Words to know

Atmospheric: Relating to the atmosphere of Earth.

Optical: Relating to sight.

Phenomenon: A remarkable person, thing or event.

Refraction: The change in direction of a wave of light as it passes from one medium to another.

Tangible: Capable of being perceived by touch.


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

We see rainbows because of the refraction and reflection of light. 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. Violet is at the bottom because the violet wavelength of light is the shortest and bends the most.

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.

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. When this happens, the secondary rainbow — the one on top — displays the colors of the rainbow in reverse order. Violet is at the top, then indigo, blue, green, yellow, orange and red. 

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. These two conditions can exist 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. 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. It occurs for the same reason as a rainbow except the light source is the moon instead of the sun. 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 is much more dim than sunlight.


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