Why can't we put metal in microwaves, but many microwaves have metallic walls/roof?

When you put metal objects in the microwave, they function as an antenna, conducting electricity. Since most microwaves are putting out around 800–1500w, that’s a lot of electricity to put into a piece of foil (or a fork).

The upshot of this, is that if you can ground the antenna, that will drain the current out of the metal, which is the purpose of the sheilding around the microwave oven’s cavity. The door with the little holes in it, is also a shield, with the little holes letting light, but not microwaves, out.

Makes sense, thank you.

A microwave works by radiation. (Microwaves are a form of radiation… Not talking about the device here). If you reflect radiation, unexpected and bad things can happen.

More here http://en.wikipedia.org/Microwave

@bomyne I am studying to work with radiation, specifically x-rays. It is reflected all the time, and nothing unexpected happens.

@FireMadeFlesh not all forms of radiation are the same. (the difference between X-rays, MRI, PET scans, CT and Radiation therapy for cancer are great examples).

The basic precept of the microwave function in heating and cooking food is the rapid mobilization of water molecules. These molecules are excited to the pint of moving at a rapid rate of speed thereby creating friction between themselves and the surrounding matter which produces high amounts of heat in a very short time.

Actually, the “metal” walls of your microwave are coated with a thin layer of a non-metallic substance. As long as the substance is there the microwave heats the food. But scratch the surface of the coating and you will get lightning and smoke instead of a nice warm dinner.

I found this out the hard way because my son created just such a scratch, and darn it, the replacement microwave had never been as good as the old one.

@Dr_C Thank you, but none of that is new information for me.

@Darwin GA, thanks!

I can’t say for certain, because I’m not familiar with how microwave radiation interacts with specific materials. I can say that the photoelectric effect (The only thing Einstein ever got a Nobel for) says that when any material absorbs a photon of the right frequency (yes, yes, I’m mixing the wave and particle properties of light, but it does that on it’s own) individual atoms can have their electrons excited, causing a voltage across a material, and a current flow due to the voltage, which means arcing, sparking and smoke. It’s actually quite possible to put (certain) metals in a microwave safely—depending on what you mean by safely. If you have the right set-up you can quite safely melt significant quantities of silver in microwave.
As far as the metal walls/roof, I believe Darwin is correct, but the entire assembly may act as a Faraday cage as well (and maybe a ground? though I doubt it), to prevent too much microwave radiation from leaking out. I’ve never specifically covered how faraday cages interact with electromagnetic radiation (as opposed to fields), but based on my understanding of E&M fields, and the nature of EM radiation: I’d say that a Faraday cage capability of blocking radiation is related to the wavelength and amplitude of the radiation. The floor, ceiling, and walls of a microwave are all metal, and have you ever looked closely at the front panel/window? The ones I’ve looked at have always had some kind of grate or plate with holes in it. My guess is that this grate is also metal and with the rest of the microwave forms a Faraday cage capable of blocking any detectable amount of radiation leakage.
All that said, a fair amount of science goes into designing microwaves, though you’ll probably never notice it. Their cavities are designed to set up a standing wave pattern of microwave radiation, causing your food to cook more evenly. (You can actually observe this pattern, with the right supplies. I’ve heard that a pan of mini-marshmallows will show a rectangular grid of melted/gooey marshmallows, a grid that will correspond with the antinodes of the standing wave pattern.