What causes a rainbow?

01.04.2015 |

Episode #4 of the course “Science questions everyone should know how to answer”

Although there’s no pot of gold at the end, rainbows are still pretty amazing things. Bands of light that appear in the sky exhibiting a range of colors, and on rare occasions two at a time—that’s a sight everyone can appreciate. But what causes a rainbow? There are three factors:

1. Sunlight

2. Water

3. Humans

Let’s start with sunlight. The sun emits wavelengths of light, only a small section of which we can see. That section or “spectrum” of light that is visible to humans is made of the colors of the rainbow: red, orange, yellow, green, blue, and violet. But we don’t see those colors shooting out of the sun at all times, do we? That’s where water comes in. When those waves of light transfer from one medium to another, such as from air to water, the color waves refract or change angles. Because every color of the rainbow has a slightly different-sized wavelength, each refracts at a slightly different angle, thereby allowing us to see them as separate colors.

Interestingly, rainbows do, in fact, form complete circles, not just the half circles that we see. Where’s the other half? Under the earth, out of sight. The angles of refraction for each color range from 42 to 40 degrees. These angles form a complete circle, but due to the fact that humans have to stand on earth instead of living in the dirt, we can only see the top half.

Another part of being human that affects our perception of rainbows is our inability to see violet well. If you ask most children what colors they see before they actually learn the colors of a rainbow, they’ll leave out violet and just say red, orange, yellow, green, and blue. This is simply because humans do not perceive violet well. It’s there, but we just don’t see it.


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