How Things Work

Lots of questions about microwaves - do most people consider them magic heating boxes?

1551. Upon removing a cup of coffee I'd heated for one minute in a microwave oven, I noticed a small ant running about, apparently unharmed. Curious, I gave it another one minute ride and when the door was open, it was still running about. How come an ant is apparently unharmed after two minutes in a microwave? -- KMB

Most likely, the ant never left the floor or walls of the microwave oven, where it was as close as possible to those metal surfaces. The six sides of the cooking chamber in a microwave oven are made from metal (or painted metal) because metal reflects microwaves and keeps them bouncing around inside the chamber.

Metals are good conductors of electricity and effectively "short out" any electric fields that are parallel to their surfaces. Microwaves reflect from the metal walls because those walls force the electric fields of the microwaves to cancel parallel to their surfaces and that necessitates a reflected wave to cancel the incident wave. Because of that cancellation at the conducting surfaces, the intensity of the microwaves at the walls is zero or very close to zero.

The ant survived by staying within a tiny fraction of the microwave wavelength (about 12.4 cm) of the metal surfaces, where there is almost zero microwave intensity. Had the ant ventured out onto your cup, it would have walked into real trouble. Once exposed to the full intensity of the microwaves, it would not have fared so well.