Breaking News: The Sky Is Blue
By Davide Castelvecchi
Every child knows that mixing blue and yellow paint gives you green. Your TV screen however, does not have yellow pixels, and it displays yellows by a combination of greens and blues. Though I never seriously tried to figure out why, this fact had always puzzled me, since yellow is supposed to be a pure color in the spectrum, not a combination of other colors.
The answer suddenly dawned on me while I was reading an article by my AIP colleague Ben Stein in July (Ben’s story by the way was picked up by Ken Chang in the New York Times). Far from being blue, Ben wrote, the sky is a complex color that “tickles our eye’s cones in the same way as does a specific mixture of pure blue and white light.”
Different combinations of colors tickle our eyes the same way? Of course, I should have known this all along! It’s a matter of information content. Light of a given color may composed of many different pure colors from the spectrum, just like the sound of an orchestra can include a number of different notes. The cells in our eyes’ retinas, on the other hand, allow us to perceive color in three types, corresponding roughly to the colors red, green, and blue.
A composite color contains a potentially infinite amount of information (the different amounts of the infinite colors in the spectrum), while the eyes essentially boil that down to three numbers.
Our eyes cannot tell the brain “I am seeing a color that’s 10 percent a certain yellow, 60 percent a particular green, equal parts of certain magentas and reds,” and so on. All the three kinds of cells can do is fire signals to the brain that represent a combination of red, green, and blue. When we see yellow, our brain gets a combination of green and blue, but it has no way of distinguishing true yellow from a red-blue mix.
The science here isn’t new at all, but you’ll find even physicists and astronomers who never figured that out.
By a happy coincidence, I got to get a real-life taste of the quirks of color perception when I visited the Exploratorium in San Francisco in December. I was there to chat with their senior scientist, the volcanic Paul Doherty. When I mentioned my chromatologic epiphany, Paul — a true guru of this sort of stuff — must have thought, “duh!,” although he mercifully didn’t say it. Instead, he took me to see one of their exhibits. It was a panel with small translucent disks lit from behind with light of different colors. The disk in the middle, Paul explained, was glowing a pure (spectral) yellow. Around it, in a circle, there were many disks that glowed with different shades of green, yellow, and brown. Those, Paul said, were each a combination of a pure green and a pure blue. One of them looked to me virtually the same color as the one in the middle. I could now see it with my own eyes: two different kinds of light that my eyes and brain interpreted as the same color.
The caption on the panel read something like: Which one of the colors looks most similar as the one in the middle? The most amazing thing is, different people give different answers.