Prisms

Completed 2019-05-21. Available releases:

Isaac Newton gets a lot of credit for playing around with prisms. For example, prisms can split sunlight, and Newton showed that this is because sunlight contains the rainbow, not because the prism colors it, as was thought at the time. What has been done can in-principle be undone, so one can imagine recombining the rainbow to get white light again, and indeed Newton did this in a later experiment.

If you Google "prism recombination", you'll see countless diagrams where they take a second prism, capture the rainbow with it, and get a white beam coming out the other side. All of these are diagrams because it turns out if you actually do that, you will not get a white beam coming out the other side—that is, all those diagrams are dead wrong. It's doubly distressing since at-least some of these diagrams are from educational websites.

One way you can actually recombine a rainbow is to add a lens to the setup, as XKCD correctly demonstrates, and this is in-fact what Isaac Newton did (albeit a lens does not (and likely cannot) converge it in the exactly correct way). If you think about it, this makes sense: optical systems are reversible. The rainbow coming out of the first prism is spreading out, so to reverse the system in the second prism, the rainbow coming in must be converging. A lens can (more-or-less) do this. But, just passing in the expanding rainbow clearly won't work, since it's expanding. If you pass in an expanding rainbow, in-fact, the rainbow spreads out even more.

Anyway, after explaining this on the ToughSF Discord, I decided to draw a similar setup by hand. This image is the result. I used no artistic references whatsoever (that is, all light paths were computed by my intuition, with occasional help from a ruler+protractor). I started by drawing the prisms and envelopes of the light paths, then sketched in the beams with white charcoal and colored pencil. I estimated the refraction directions instead of calculating them, so no particular material is implied, although I tried to make it roughly consistent with some kind of glass. The background was black charcoal, which I smeared copiously with a paper towel. I did minor highlights with gel pens and more white charcoal.

After scanning and uploading, I did post-processing to clean it up and improve it some. Photoshop has moved to some garbage subscription model, and you can't even download previous versions (I had CS 5 or 5.5 or something on my old laptop). Today, you might even get your pants sued off if you even have the old software (No; you don't own software you bought. Why would you think that?). So, I looked into a permanent replacement from a company/entity that isn't lawful-evil. GIMP doesn't support lossless filter editing, which is super mandatory, and it's pretty buggy. So, I tried out Krita. Krita is a bit clunky, and the filters weren't as easy to use as I'd have liked, but it got the job done. I will be using it as my heavy-image-editing program for the foreseeable future.

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