Everybody’s favorite — microwaving a lightbulb.  Pretty!

At least two posts suggest that if you put the bulb in a mug of water (with the bulb part sticking up) then it won’t explode.  I believe that’s because the water acts as a dump for the microwave energy, keeping the bulb from heating up out of control.

What’s going on?  This is similar physics to two of my previous posts (microwaving a CD, and the microwaving a grape post).  Lightbulbs are a partial vacuum inside, but also have a small amount of gas, usually Argon.  The microwave makes a current in the metal of the bulb (just because microwaves push charges around).  This current lights up the filament in the light bulb.  But it doesn’t glow in the way that you’re used to seeing a lightbulb glow because the high voltage creates a plasma inside the lightbulb (remember, this is an ionized gas), and the gas glows purple.  I’m pretty sure that this doesn’t require the lightbulb filament — in other words, this should work with a burned out bulb.

The light pulses because the magneton that creates the microwaves in your microwave oven is pulsing on and off.  I’m not quite certain why there are the different colors created, though it seems to be related somewhat to the rotation of the bulb in the microwave.  It may simply be that at different angles, the lightbulb acts as a stronger or weaker antenna for the microwaves, creating more or less voltage, and thus a plasma that glows at different colors as more or less energy is dumped into it.

I never found a good comprehensive site with a succinct explanation, but this page has some interesting discussion.

This is actually what normally happens in a regular fluorescent tube light (that is, an ionized gas glows), so if you put a fluorescent tube light in the microwave, it should glow like normal.

A friend pointed me to this wonderful set of videos, “Is it a good idea to microwave this?”

His favorite, he says, is the giant mercury lightbulb.