Grandma's Microwave Oven

My Grandma has an (older-type) microwave oven that has no glass in the door, but has instead a metal 'grill' that has thousands of tiny holes in it - so how come the microwaves don't 'escape' and get out through the holes ?
- Peter (age 14)
Qld. Australia

The microwave frequency of most microwave ovens is 2.45 gigaHertz which gives an equivalent  wavelength of about 12.24 centimeters.  A thin conducting metallic screen a few tenths of millimeters thick having lots of millimeter size holes is an effective shield and will not let the microwaves out.  You can think of it as that the big waves can't get throuh the small holes.  The technical term is 'wave-guide below cutoff'. Since microwaves easily penetrate glass there is really no difference in electromagnetic shielding between an oven door with or without glass.  The only real improvement in glass shielding is from possible splatter of a heated liquid.
So Grandma's microwave oven is probably safe.

LeeH

(published on 04/22/2008)