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Green, the fourth primary color?

The questions that drive scientific research seem to strike when you least expect.

I took my two children to Legoland in Carlsbad, California recently. As we approached the entrance, I noticed four giant Lego blocks greeting us; they were red, yellow, blue, and green. This got me thinking about colors.

Yes, there is an additive system of three primary colors — red, green, and blue: the colors of light — but let's set that aside for a moment. In the subtractive system (the one that dominates our kindergartens), the three primary colors are red, blue and yellow; and their associated secondary colors are orange, purple, and green. Yet, whenever four colors are displayed, green is the fourth color — open up a small four-pack of Crayola crayons to see for yourself. One rarely finds red, blue, and yellow displayed with the other two secondaries: orange or purple. Why is this?

The retina has cone-like structures that are receptive to light. The cones are labeled according to their sensitivity to wavelengths. They are called short, medium, and long cones. Studies have shown that the peak sensitivity of long cones is in the greenish-yellow region of the spectrum; and both the short and medium cones are also highly sensitive to green. So, by and large, we are very sensitive to green. So perhaps that is why, when four colors are displayed, green is often the fourth. Green is simply more attractive to our eyes.

But why are we sensitive to green? One can only guess. Foliage is most often green, so one would imagine that early man would have to become very sensitive to the various hues of green to determine if any danger lurked in the dark or light green underbrush. Anyone without sensitivity to green may have been gobbled up. There may be a more substantive reason, but the balance is interesting.

But then one asks why, of all the colors, is it green that is reflected by chlorophyll? I don't know; but the point in science is to ask, to ponder, to wonder.


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