Jim Pivarski

In grad school, I was a teaching assistant for a course called, Why the Sky is Blue. It was a qualitative introduction to physics for non-majors, covering a lot of the same topics as Physics I, such as forces, conservation of energy and momentum, electric charges and magnetic fields, in less detail, with not much math. The actual question about why the sky is blue was saved for the end. As the course dragged on and the students (who expected no math, rather than not much math) started to complain, "Are we ever going to find out why the sky is blue?" I watched the schedule slip and wondered the same thing. We skipped some sections and managed to wedge it into the last lecture: finally, we were talking about why the sky is blue! "The sky is blue because of Rayleigh scattering." Okay, that's not an answer we hadn't defined Rayleigh scattering, there wasn't time for it, so we said that air molecules absorb and re-radiate—effectively changing the direction of—blue light more than red light. Red light goes straight through the atmosphere, and blue light bounces around, making the whole sky glow blue. Conversely, sunrises and sunsets are red because you're looking at the light that has gone straight through a larger wedge of atmosphere. It lost most of its blue on the way to your eye. Pretty good explanation, for not being able to say I=I01+cos2θ2R2(2πλ)4(n2−1n2+2)2(d2)6 (the 1/λ4 part affects small-λ blue light more than large-λ red light). We also showed pictures like this sunset: to demonstrate the effect of straight-through red light and bouncing-around blue light. So in the end, "Why is the sky blue?" Answer: "Because sunsets are red!" "And why are sunsets red...?" It was understandably unsatisfying. One thing was only explained in terms of another thing. But even if we had the time to get into detail about Rayleigh scattering, they could reasonably ask, "Why does light scatter according to that formula?" We could go deeper and explain Lord Rayleigh's proof in