BOB HIRSHON (host):
Metal in microwaves. I’m Bob Hirshon and this is Science Update.
It’s the cardinal rule of microwave ovens: don’t ever stick anything metal in there. So why do some microwaves have metal racks these days? We asked University of Virginia physicist Louis Bloomfield.
LOUIS BLOOMFIELD (University of Virginia):
Metal and microwaves are not incompatible; in fact the entire box within the microwave is all metal. So when the microwaves encounter one of these thick metal walls, they reflect the microwave the same way a metal mirror reflects light.
Food heats up as it absorbs energy from all those reflected microwaves. Bloomfield says the metal racks used in microwaves are thick and blunt enough to safely reflect microwaves as well. But problems occur when metal objects are thin and sharp, like twist ties and crumpled aluminum foil. In microwave ovens, electric currents concentrate at the sharp edges of these objects, until they discharge, resulting in sparks, and sometimes, fire. I’m Bob Hirshon, for AAAS, the science society.