The world is awash with the color purple — lavender flowers, amethyst gemstones, plums, eggplants and purple emperor butterflies. But if you look closely at the visible-light portion of the electromagnetic spectrum, you’ll notice that purple (which is different from bluish hues of violet and indigo) is absent.
That’s because purple may be made up by our brains; It exists only because of how the brain processes color.
So does that mean purple doesn’t really exist? Not necessarily. The answer lies within the mind-boggling way that our brains perceive and combine different wavelengths on the visible light spectrum.
“I would actually say that none of color actually exists,” said Zab Johnson, an executive director and senior fellow at the Wharton Neuroscience Initiative at the University of Pennsylvania. “It’s all the process of our neural machinery, and that’s sort of both the beauty and the complexity of it all at the same time.”
All color begins with light. When radiation from the sun hits Earth, a range of wavelengths are present. There are long wavelengths, like infrared rays and radio waves, and shorter, high-energy wavelengths, like X-rays and ultraviolet rays, which are damaging to our bodies, Johnson told Live Science.
Toward the middle of the electromagnetic spectrum lies visible light — the light our brains can see — which represents only about 0.0035% of the electromagnetic spectrum. This is what we perceive as the colors of the rainbow. On one end of the spectrum are longer wavelengths, which we perceive as red, and on the other are shorter wavelengths, which we perceive as indigo and violet.
Our perception of color involves specialized receptors at the back of our eyeballs, called cones, that detect visible light. Human eyes have three types of cones: long wave, mid wave and short wave. Each is sensitive to particular wavelengths. Long-wavelength cones take in information on reddish light, mid-wavelength cones specialize in green, and short-wavelength cones detect blue.
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When light hits our eyeballs, these three receptors take in information about the light and their respective wavelengths and send electrical signals to the brain. The brain then takes that information and makes an average deduction of what it’s seeing.
“Our machinery is sort of doing this complex sort of calculation of these three different ratios all the time,” which forms our perception of color, Johnson said. For example, if long-wavelength and mid-wavelength cones are triggered, the brain infers that we’re seeing orange or yellow. If mid-wavelength and short-wavelength cones are activated, the brain will make a conclusion of teal.
So what about purple? When short-wavelength (blue) and long-wavelength (red) cones are stimulated, your brain “makes something that’s actually not out there in the world,” Johnson said. Red and blue are on opposite ends of the visible spectrum: When the brain encounters these wavelengths, it ends up bending this linear visible spectrum into a circle. In other words, it brings red and blue together to make purple and magenta, even though that’s not what light is really doing.
As a result, purple and magenta are known as “nonspectral” colors, because they don’t really exist as actual electromagnetic radiation. Nonspectral colors like purple are made of two wavelengths of light. In contrast, spectral colors — red, orange, yellow, green, blue, indigo and, importantly, violet and indigo — are made of just one wavelength.
Regardless of its physical existence, purple has captivated people for millennia, noted Narayan Khandekar, director of the Straus Center for Conservation and Technical Studies at Harvard Art Museums. For example, ancient Phoenicians ground up sea snails to make a color known as Tyrian purple, which was reserved for royal or ceremonial robes. Today, purple is still often associated with wealth, power and even magic. “So that connection still exists, even though there are other versions of purple available now,” he told Live Science.
So, whether manufactured in our minds or made from ground-up shellfish, purple is unique and deserves closer look. “It doesn’t really exist in nature. And so when you can create it, it has this extra value,” Johnson said. “Now purple is even more special.”
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