Have you ever wondered why the sky is blue? A professor in Georgia says it's not - it just looks blue to humans.
"Blue skies, Smiling at me, Nothing but blue skies...Do I see..."
This classic American song revels in a sight we have all enjoyed on a sunny day, but it fails to mention that our vision deceives us.
The blue skies we see are not truly blue at all.
When it comes to explaining why the sky appears blue, the way our brains interpret color plays as big a role as the physics of light. Most scientists overlook this, but not Georgia Institute of Technology professor Glenn Smith. In the July issue of the American Journal of Physics, he brought together research from physicists and vision experts to form a comprehensive explanation.
He says the important thing to remember is that light does not come in colors - it comes in wavelengths. Color exists only within our minds.
"Think of a water wave, like one you might have seen out on the ocean," he said. "If you look at that wave, there are crests and troughs, and if we measured the distance between two consecutive crests, say, we would call that the wavelength of that water wave. Now, a light wave is something like a water wave, only the wavelength is much smaller."
The human mind perceives different wavelengths of light as different colors. Red is the longest wavelength we can see, and blue and purple are the shortest. There are also wavelengths that are invisible to us, like the radio waves that carry this broadcast and the ultraviolet light that can cause sunburns.
When light from the sun strikes earth's atmosphere, some of it is scattered by gas molecules. Short wavelengths, which we interpret as blue, are dispersed more than long wavelengths, so we see blue wherever we look in the sky. The question is why don't we see the even shorter more abundant wavelengths as purple. Scientific instruments show that they are there, so one would think the sky would be purple. Mr. Smith says this is the problem with relying on physics alone to explain a blue sky.
"I realized that there was a little more to the story than that," he said. "You know, your eyes have something to do with it."
In the eye, special light-sensing cells called cones send signals to our brains that enable us to see color. Cones come in three types, depending on whether they respond to short, medium, or long wavelengths of visible light. We need specific responses from all three types of cones to distinguish color. Mr. Smith says that the way our brains interpret these signals is what prevents us from seeing the violet in the sky.
"We have in our brain something we associate with blue, and that's a particular wavelength light," he explained. "We see exactly the same thing as when we look at the sky light, so we naturally call that sky light blue. Even though there's violet light there, the cones are kind of weighting that response, and that weighting turns out to give you something that's equivalent to blue and not to violet."
The human mind interprets the mixture of wavelengths in the sky as blue, but other animals see colors we cannot even imagine.
"We know that some insects see a different spectrum of light than we do," he said. "They don't see the long wavelengths that we do - the reds - but they see shorter wavelengths than we do. They can see ultraviolet. So when the honeybee looks up at the sky, they see ultraviolet light that we can't see."
Unlike the bees, we humans will have to be content with seeing blue skies.