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The official colors of Carroll College are purple and gold: Unfortunately, neither of these is really a color. Gold is a metal, although yellow is a color. Sadly, purple isn’t a color either. The next time you see a rainbow, look carefully, and you’ll find that purple is missing.

Rainbows have all colors of the spectrum: red, orange, yellow, green, blue and violet. But rainbows never include the color purple, because purple is not a true color. Why not? And if purple is not a color, what is it?

More than 300 years ago, Isaac Newton did a series of experiments with prisms. He found that when a beam of white sunlight passed through a glass prism, the light broke up into a rainbow of colors. But even more interestingly, he found that if you took this rainbow, and put it into another prism in the right way, you would get a beam of white sunlight again. Newton discovered that “white” is not a true color. White is a mixture of all the colors put together.

So what are colors? What makes red different than green or blue? Light is a wave, and the length of a light wave, the distance from one peak to the next, determines its color. All light waves are really short, just a few thousand times bigger than an atom. However, some light waves are longer and others are shorter. The longest light waves that we can see are red. Slightly shorter waves are orange, then yellow, then green, then blue, and the shortest light waves that we can see are violet. 

Purple is a mixture of colors, like white. If you mix blue light and red light, your eye will see purple, but in reality, it’s just a mix of blue and red. Why does the human eye see purple as if it was a real color, like green or yellow? To understand this, we need to take a voyage into the human eye.

At the back of the eye is an amazing tissue called the retina. The eye projects an image of the world onto the retina, where special cells receive the light and send signals to the brain. Our color vision comes from certain cells called cone cells. There are three types of cone cells. One reacts mostly to red light, another reacts mostly to green, and the third reacts mostly to blue. That’s why red, green and blue are the primary colors: The human eye can really see only these three colors. 

So how do we see the color orange? If the brain receives signals from lots of red cone cells and some green cone cells, we see orange, because orange is closer to red than green. The human eye can’t tell the difference between a beam of pure orange light and the right mixture of red and green. The difference is that true orange light would not be affected by a prism, but a prism would separate a mix of red and green.

Scientifically, purple is not a color because there is no beam of pure light that looks purple. There is no light wavelength that corresponds to purple. We see purple because the human eye can’t tell what’s really going on. Of course, we can still use purple as a color in art or on our Carroll Fighting Saints football uniforms. But if a flying saucer were to land in Helena tomorrow, the little green men inside would probably not have a word for “purple” although they might have words for red, orange, yellow, green and blue.

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Hopefully the little green men will still be football fans.

Kelly Cline, Ph.D., is associate professor of astronomy and mathematics at Carroll College.

 

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