Hacker Friendly

I’ve just finished watching all of Walter Lewin’s exceptional MIT course 8.02: Electricity and Magnetism. In lecture #31, he gives an hour long lecture about rainbows. I highly recommend watching it, even if you don’t have time to work through the rest of the course. You’ll learn all sorts of fascinating things about the phenomenon of the rainbow. For example, everybody knows that the colors run red-orange-yellow-green-blue-violet. But is red on the inside or the outside of the rainbow? Is this always true? Are some parts of the rainbow brighter than others? Are rainbows polarized? How big would you expect them to be? And why do they happen at all?

One particularly useful question is: where would you expect to see them when you look up at the sky? That answer is pretty easy. If the sun is out, look at your shadow. The sun should be behind you. From the shadow of your head, look up (or down) about 42 degrees. If there are fine water droplets there, then you might see a rainbow.

I was thinking about this a few days ago while standing on this corner when I happened to notice a rainbow in the sky. It was about 4:30 in the afternoon. Three things surprised me about this rainbow.

First, it wasn’t raining. But that was no problem, because there were lots of very low, very fine clouds in the sky.

Second, it wasn’t a whole rainbow, but was just a small piece of one (like a sun dog). But that might have been explained by the variable amount of water in the sky. It was, curiously enough, roughly in the shape of a rectangle.

Third, and most unsettling: it was very near the sun in the sky! I thought, “my God, physics is broken and Walter Lewin was wrong!” How could the rainbow be right next to the sun if it should be 42 degrees above my shadow, which was pointing in the opposite direction?

Obviously, I must have made a mistake, because Walter Lewin couldn’t possibly be wrong about rainbows. But there was this strange rainbow in the sky, plain as day, just to the right of the sun. So I decided to do the find-a-rainbow trick in reverse: with the rainbow to my back, I looked at where my shadow should have been (as if it were the sun) and looked up 42 degrees.

There was the space needle.

I thought, “Aha, the sun must be reflecting in one of the windows.” But the windows of the bottom half of the space needle are pointed down, not at the sky. Then I noticed that one of the windows was very shimmery, as if it was a billowing piece of bright plastic.

Then it hit me– that window was reflecting the sun, bouncing off the waves in the sound! Incredible! Physics works after all!

That also explained why there was just a rectangular sliver of a rainbow: that was the shape of the light making it.

I turned around again to examine it more closely. The colors were in the correct order (red on the outside) and it was slightly brighter on the inside of the arc. After a few moments, the rainbow suddenly disappeared. Enough time had passed that the angle of the sun had changed, and that window was no longer illuminated by the waves.

As Walter Lewin says, many people have looked at rainbows, but few people have actually seen them. I feel like I have had a chance to actually see a rainbow, thanks to an accident of the universe and a crazy Dutch mad scientist from Massachusetts. If you have any interest in electricity, magnetism, resonance, unusual optic or auditory phenomenon, or even cosmology, you will certainly enjoy MIT 8.02.