What keeps microwave radiation from leaking out the oven door?
If you ask me, one of the cleverest parts of a microwave oven is the door. It lets you see into the oven to make sure your potato isn't about to explode, yet keeps your face from being cooked off by microwaves in the process. It accomplishes this remarkable feat with a metal mesh-basically a sheet of metal with a regular pattern of small holes drilled or punched in it. This, along with the metal material of the oven cavity itself, forms a sort of reverse Faraday cage. A Faraday cage is a metal enclosure that normally keeps electrostatic fields and RF (radio frequency) radiation from getting in, but in the case of the oven, from getting out.
A solid sheet of metal would be the most efficient at containing RF, but since you can't see through metal (unless you happen to be Superman), a compromise must be made. How much RF escapes through the mesh depends largely on two factors: the wavelength of the radiation in question, and the diameter of the holes. (For you picky types, the conductivity of the mesh material has a slight but negligible effect, as does the shape of the holes, among other things.) The shorter the wavelength of the RF, the smaller the holes need be to block the radiation. Microwave ovens operating at 2450 MHz emit radiation with a wavelength of about 12 centimeters, and the holes in the mesh of the door are typically on the order of 1-2 millimeters in diameter. Since the holes are small compared to the wavelength of the microwaves, little radiation can leak out. There are also mesh screens on the sides of the oven cavity, one to protect the oven light while allowing it to shine into the cavity, the other to permit ventilation.
The FDA has set emission standards for new microwave ovens at a power density of 5 milliwatts per square centimeter at a distance of 5 centimeters from any point on the outer surface of the oven. Typical new ovens have a leakage of only 0.2 mW/cm2 at that distance-a tenth of the FDA standard. Dirt, mechanical abuse and everyday wear and tear can eventually reduce the effectiveness of the door seals, thus increasing leakage. If you're concerned about emissions from an older oven, you can purchase leakage sensors at many hardware and home improvement stores or online. Keeping the seals on the oven and door clean, as well as opening and closing the door gently, will help keep your oven safe to use. So go ahead and ogle your latest culinary masterpiece while it cooks. Your food will cook, but your head will not. Or so one certainly hopes.