03 March, 2010

The Sun is Green

Okay, so the sun doesn't look green. But appearances can be deceiving. After all, everyone knows it's not yellow, even though that's the color that schoolchildren always use in drawings. Common wisdom says it's white—but what is white?

Click to enlarge.

White light is the combination of all colors of light. The sun is white because it emits all colors of light in our visible spectrum. Yet this brings up a question we rarely think about: is white a color?

If white is the combination of other colors, and black is the absence of other colors, can they really be considered colors at all? It's hard to answer this, because it depends on your definition of color. Color is defined so loosely that speaking clearly about it is almost impossible. It is better to speak of hues, such as red, green, purple, and orange. White is not a hue; rather, each hue can have a lighter tint or a darker shade to it. Orange, for example, is my favorite hue, but only when it has a particularly light tint to it. Dark orange shades seem like an entirely different color.

Of course, "white" is a term which corresponds to some sensation in the brain which we usually call color. But when looking at the different tints and shades in the image above, it becomes easy to see that white light is in fact some hue or combination of hues that has very low saturation and a very, very light tint. And if this is true, then the only question becomes whether sunlight contains one or more hues in greater concentration than the others.

In fact, sunlight does just that. The wavelength of sunlight varies throughout the visible spectrum (and more), but it is highly concentrated at the 500 nanometer wavelength: green.

Notice the peak at 500 nm.

So although sunlight appears white, the main hue it is consisted of is green. It just happens to be a very, very lightly tinted green of almost no saturation. Oh, and some other colors thrown in as well, but in far less of a concentration.

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