Why the Sky Isn't Violet

Think again if you believe that a degree from MIT will help you answer many of the questions that children ask about their environment.  In a recent article in the Boston Globe, Anthony Doerr tells of his brother Chris, who was asked why the sky was blue during his oral doctoral examination at MIT sixteen years ago.  He didn’t know the answer. Doerr points to many before Chris who did not know the answer either—Aristotle, DaVinci, John Tyndall, just to name a few (though they all tried).  However, Doerr does find some help for them:

If only Tyndall could have put his hands on an advance copy of science historian Peter Pesic's luminous new book, Sky in a Bottle.

In 10 cleanly written, well-paced chapters, Pesic traces the progression of our understanding of atmospheric hue through dozens of scientists, philosophers, and artists. His smartest move was to structure Sky in a Bottle like a mystery story, coaxing us puzzle by puzzle through the (very) uneven advances of knowledge over the centuries. You find yourself forgetting what you think you already know. What is air? What is reflection? Is light a particle or a wave, anyway? Why is the sky red at sunset? What role, if any, does interference play?

If science is a systematic search to understand the universe, understanding why the sky is blue is perhaps the most accessible question the universe can offer. It's also a beautiful illustration of how the very largest things in our world are affected by the very smallest. Without atoms, our sky isn't blue. Indeed, except perhaps for our bodies themselves, the color of the sky might be the quintessential example of how excruciatingly small things— molecules, and the atoms that make them up—manifest themselves in our physical reality.

So do you want to know why the sky looks blue? First, remember that the wavelength of blue light is shorter than that of red light. Second, remember that the atmosphere is molecular; it (primarily) consists of molecules of nitrogen and oxygen. Third, remember that I'm a novelist by profession and barely know what I'm talking about.

Regardless, here's your one-sentence answer: Since the wavelength of blue light is short enough to interact with molecules of oxygen, it gets scattered all over the sky and reaches our eyes from all sorts of different angles.

Read the entire article here.