Cosmos Reboot: On Light

Another Monday, another recap of a new episode of the Cosmos reboot. This one was all about optics, and much of it was excellent. This was in part due to the fact that its first couple of historical segments focused on non-Western figures, and I don't know as much about their background to be able to nitpick. First up was Mozi, a Chinese philosopher from circa 400BCE, who may have been the first to demonstrate the camera obscura technique of projecting images from a pinhole in the wall of a dark room. He was followed by ibn al-Haytham, circa 1000CE, who did the first fairly complete analysis of the image formation. Interestingly, while the show made a point of condemning the Qin dynasty for its outlawing of Mohism and the burning of books, they praised the Caliphate of al-Haytham's day for preserving knowledge without going on to condemn later regimes for backsliding into fundamentalism. I guess the rah-rah equating of science with a rather modern conception of freedom of thought only extends so far.

Then we had the obligatory Newton moment, looking at the dispersion of light by a prism, with an odd little coda that seemed to fault Newton for not doing experiments that wouldn't've made sense for him to try. The tease after this was that the next step would jump to around 1800, so I thought I knew what was coming next, but instead we got another shout-out to William Herschel (whose animated incarnation looks sort of addled...) for the discovery of infrared light through an experiment with thermometers. Then it jumped to the positively Dickensian life story of Joseph Fraunhofer, who was the first to observe absorption lines in the spectrum of the Sun, kicking off spectroscopy as a science and eventually leading to quantum mechanics.

This was one of those gaps that make this show so frustrating. When they said they were going to 1800, I was expecting to hear about Thomas Young, the British polymath who provided the iconic demonstration that light is a wave, by showing interference phenomena. It's not clear whether he actually did the double-slit experiment (cartoon version up top) that bears his name-- that might've been Fresnel, who did a lot of work really nailing down the wave theory-- but it's a critical moment in the history of optics. More important, I would say, than Herschel's discovery of infrared, especially for the story they were actually telling in this episode(*).

The omission of Young and Fresnel was especially frustrating because it made the following material more confusing than it needed to be-- in explaining Fraunhofer's importance, they went through a kind of trippy visualization of sound waves, and then jumped directly to light as a wave, without explaining why that works or how we know it. And that's kind of an important detail in the history of science. But I guess something had to be cut to make time for annoying Taco Bell ads.

There were some bits of the remainder that were a little off-- the animation of electron orbits in atoms was a little too particle-like (but that's a really hard thing to visualize, and they did about as well as I could really expect), and a couple of the statements about quantum were a little overdone-- but other than the giant glaring omission of the wave theory of light, this was a very good episode. And the closing visualization of the world in different wavelengths of light was very nice (though it could've been scored to something less cliche than "Rhapsody in Blue"...).

I thought that at some point I had posted a detailed description of the double-slit, which I could link to here as a more complete explanation of the missing physics; alas, while I have a bazillions posts about interference effects, that's not one of them. And this week is going to be brutal, time-wise, so I'm not likely to write a new one right away. But I'll throw in another plug for this totally awesome weak-measurement experiment "tracking" photons through a double-slit experiment.

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(* - I should emphasize that I'm not saying the Herschel thing wasn't cool-- it was, and it's a great lesson about the scientific method and serendipity. But at the same time, in the context of this episode, it's kind of an odd little detour-- infrared radiation doesn't end up being important to anything that follows until they get to showing the sky in different wavelength bands. It would've been much better to spend that time setting up light as a wave.

(But this is generally in keeping with their tendency to tie everything to astronomy and astronomers. And while Young was a brilliant guy who did lots of interesting stuff, he wasn't an astronomer, while Herschel was.)