Avoid clichés like the plague, writing books tell you, and ordinarily I do my best to do so–but this week, I can’t help but start out with a cliché, one of the oldest in the book, the question every small child supposedly asks at some time or other: “Why is the sky blue?”
It might seem an odd choice, since surely (you’re probably thinking) this has been settled long ago. But while it’s true there’s a standard explanation, there’s actually more to the question, scientifically, then a four-year-old knows to ask. In scientific terms, the question isn’t “Why is the sky blue?” but “Why is the sky blue…and not purple?”
The answer has been spelled out fully for the first time in a new scientific paper in the July issue of the American Journal of Physics, written by Georgia Tech engineering professor Glenn Smith.
The standard explanation, the one everyone learns in school, is attributed to the 19th century English scientist Lord Rayleigh, so it’s been around for a while.
As prisms and rainbows demonstrate, so-called “white light” is really a mixture of all the visible colours of light.
Colour is determined by wavelength. As sunlight enters the Earth’s atmosphere, it’s scattered by the various molecules that make up that atmosphere, especially nitrogen and oxygen.
The shorter wavelengths in white light, at the blue and violet end of the spectrum, are scattered more than the longer wavelengths, at the red and yellow end. That means that when we look at the sky in any direction except directly at the sun (which, obviously, is how we normally look at the sky) it’s mostly light from the blue end of the spectrum that’s scattered into our eyes.
(Sunsets and sunrises appear red because the light of the sun, when it’s low on the horizon, has to pass through more atmosphere to get to your eye. Only the long wavelengths at the red end of the spectrum can make it through.)
That’s all fine, as far as it goes, but in fact the light in the sky is a mixture of violet (at around 400 nanometres) and blue (at around 450 nanometres). So why doesn’t the sky appear violet, or at least blue-violet?
What’s missing from the standard explanation, Smith points out in his new paper, is a discussion of the physiology of the eye.
The eye is sensitive to light in wavelengths between approximately 380 and 740 nanometres. The retina has about five million specialized cells called cones with which it senses color. They come in three types, sensitive to long, medium and short wavelengths. (There are also about 10 million rods, which are sensitive to lower levels of light than cones but don’t distinguish colours.) The cones show peak responses at 570 nanometres (yellow), 543 nanometres (green) and 442 nanometres (between blue and violet) respectively, but they’re at least somewhat sensitive over much broader ranges of wavelengths, which overlap each other to a certain degree.
As a result, more than one mix of wavelengths (spectra) can produce the same response in the eye. For example, pure yellow is centered around 570 nanometres. But the same set of cones that react to yellow light respond to a mixture of red and green light–which the eye thus also sees as yellow. (Two spectra that produce the same response in the cones are called “metamers.”)
The same thing happens with regard to the sky, Smith says. The sky’s combination of violet and blue produces the same response in the cones as pure blue plus white light. So even though the violet wavelengths are as present in the sky as the blue ones, to the eye, it simply appears blue.
Interestingly enough, this probably means that the sky looks entirely different to most other animals, because only humans and some other primates have this three-cone retinal system. Most animals have only two types of cones. (And then there are the honeybees and some birds, which can see in ultraviolet wavelengths that are invisible to us. Their view of the world must be very different indeed.)
So there you have it: the complete explanation for why the sky is blue. And yet, even with this information readily available, I suspect that the most common answer given to this most clichéd of questions will continue to be itself a cliché:
Namely, “Go ask your mother.”
(Source: LiveScience.)
