OK, I wouldn’t let it run quite as long as these bored college students did, but it DOES look REALLY cool (and it’s a great use of those annoying AOL CD’s, or the romantic mixes that your old boyfriend made for you):
And another really pretty one (gotta love the Darth Vader-esque breathing in the background), with an extra bonus: aluminum foil!
You can play around to see if it matters if there are images on the CD, if it looks different printed side up or shiny side up, etc. This site claims that it works best label-side up, and that the less ink on them, the less they smoke.
As you know, it’s not supposed to be good to microwave metal. That’s because the microwaves can push the electrons around in the metal. (Electrons in non-metal, or non-conductive material are kind of glued in place, so they can’t be pushed around). That can make the metal heat up (just like a metal wire will heat up when it’s conducting a current) and do all sorts of bad things to your microwave. You can read more about microwaves and what they do to metals here.
So, CD-ROM’s have a thin aluminum layer. And the microwaves push the electrons around in the aluminum, making big currents, which heat up the aluminum so much that it vaporizes (turns into steam)! The electric current is still there, though, so it jumps across the vaporized aluminum (making a pretty light show) to get to another section of aluminum. There is a little bit of similar science between this and the Microwave a Grape activity I posted earlier, in that you’re seeing air glow as electrons jump through it (a phenomenon called arcing). You’ll see a bunch of little paths burned into the aluminum after a while. An interesting observation from this site:
Some of the islands will be shaped so that they make very good microwave antennas. These spots will focus the microwave energy, and get very hot. Now you will see just a few bright spots spewing a lot of smoke. The good part of the light show is over, turn off the oven.
Here is a lovely image of a CD post-microwave, showing beautiful fractal trees where the electrical arc made its way across the aluminum.
I’m still a little confused as to why the patterns burned in the CD follow these circumferential patterns. I imagine that the CD data is originally etched in circumferential patterns, making the aluminum thinner in these regions, and thus channeling the electricity in these circles.
For extra fun, if you happen to have a Tesla Coil lying around, here is what happens when you place the microwaved CD on top of the Tesla coil. I got this from ElectricStuff.co.uk, which has even more pictures.
I believe what’s happening is that the electric current from the Tesla is flowing just through the parts of the CD that still have aluminum on it, generating high heat and arcing in lovely patterns.