WARNING : THIS IS A RELATIVELY DANGEROUS EXPERIMENT. IT CAN SERIOUSLY DAMAGE YOUR MICROWAVE AND IT PRESENTS A REAL RISK OF AN ELECTRICAL FIRE. At the bottom of this writeup, under 'Sources and further reading', is the URL of a quick-time movie of the experiment being performed. If you're considering trying this experiment, watch the movie first so you can see what you're getting into. THIS WARNING IS NOT A JOKE.

This is one of those kitchen-science things that demonstrates laws of physics and/or chemistry using common household objects. Such experiments range all the way from weak to wonderful, and I am sure I'm not alone in thinking that there is often some correlation between how potentially dangerous such experiments are and how cool they are; i.e. riskier = cooler. Judged on those terms, this particular experiment is very cool indeed.

For this experiment you'll need a grape, a microwavable plate, a paper towel, a knife, and a microwave oven.

Place the grape on the plate and cut it in half length-wise leaving a thin strip of the grape's skin uncut, so that the two halves are still joined by a bridge of skin. Make sure the grape is placed on the plate with the cut surfaces facing upwards. Dab the cut surfaces with the paper towel to dry them off as much as possible. Now place the plate in the microwave and turn it on at full power. A few seconds later, the grape will begin to crackle and spit loudly and emit bright sparks. It may sometimes also give off rising bubbles of white-hot luminous plasma. Be afraid.

Maarten Rutgers excellent "Physics inside a Microwave Oven" page explains:

"Our suspicion is that the grape halves act as a small dipole antenna for the microwaves. Optimally such an antenna is 1/4 the size of the wavelength, which is pretty close for our grape. The skin flap is a conductor and currents run back and forth across it. As it heats up, it dries out, increasing it's resistance further, causing more heat, etc. Finally it ignites... On rare occasion we noticed rising clouds of luminous plasma (often referred to as 'ball lightning' in other microwave oven web pages)." (1)

I have tried this experiment (inspired by Infinity's writeup under microwave) but that was before I knew how potentially dangerous it could be. I have to say, the results were pretty spectacular, and given that I didn't even dry the grape first, they were probably fairly modest in terms of what's achievable. And I'm thankful for that.

    Sources and further reading:
  • (1) Maarten Rutgers, The Ohio State University Physics Department, "Physics inside a Microwave Oven" at http://www.physics.ohio-state.edu/~maarten/microwave/microwave.html#grape contains links to quick-time movies showing grape preparation and some awe-inspiring plasma clouds (click on the photos)
  • Try Louis A. Bloomfield's "How Things Work : Microwave Ovens" page at http://rabi.phys.virginia.edu/HTW/microwave_ovens.html for further discussion of the science involved.
  • Check out the amusing "Fun with Grapes - A Case Study" at http://www.sci.tamucc.edu/~pmichaud/grape/ for a well-documented lesson in how NOT to do it.