How does a microwave heat food, and why does metal spark?

A microwave oven fills its chamber with electromagnetic waves that water molecules in food absorb, and the absorbed energy becomes heat. Metal can spark because the waves push electric charge around in it, and the charge can pile up on sharp edges or points until it jumps through the air as an arc. Smooth, flat metal usually does not spark. [1] [2]

The waves come from a magnetron, a vacuum tube originally developed for military radar, according to [scientificamerican.com](https://www.scientificamerican.com/article/how-the-microwave-works/). Their wavelength is about 12 centimetres, longer than visible light but shorter than most radio waves, as [newscientist.com](https://www.newscientist.com/article/2369113-what-is-really-going-on-when-we-microwave-our-food/) notes. Physicist Aaron Slepkov told [livescience.com](https://www.livescience.com/why-metal-sparks-in-microwave.html) that the frequency is roughly 2.5 gigahertz, a value water absorbs well. TDK gives 2.4 GHz. [1] [3] [2] [4]

A water molecule is bent, with a slightly positive end and a slightly negative end. When the electric field reverses rapidly, the molecules rotate and vibrate, and friction between them produces heat. This is called dielectric heating, per [tdk.com](https://www.tdk.com/en/tech-mag/inductive/how-does-a-microwave-oven-work). Paper, glass and plastic let the waves pass, while the metal walls reflect them, according to [engineering.mit.edu](https://engineering.mit.edu/ask-an-engineer/why-cant-we-put-metal-objects-in-a-microwave). Ice heats slowly because its molecules are locked in a crystal structure and cannot rotate easily. [4] [5] [3]

Sources differ on where the heat appears. The MIT account says food may cook from the inside out, the outside in, or uniformly, depending on where the water lies. New Scientist calls inside-out heating a myth: the waves lose energy as they pass through food, so anything more than about 2 centimetres from the surface heats little. The deeper parts of thick food warm mainly by heat conducting inward from the outer layer. That conduction detail is an inference from the New Scientist point, not stated outright in the results. [5] [3]

Metal reflects most microwave energy, so food inside a foil container is shielded and does not heat well, according to [newscientist.com](https://www.newscientist.com/article/2369113-what-is-really-going-on-when-we-microwave-our-food/). The waves also slosh electrons around on the metal's surface. On a smooth surface this causes no trouble, but at edges such as fork tines the charge can pile up into a high voltage, as Slepkov explained to [livescience.com](https://www.livescience.com/why-metal-sparks-in-microwave.html). A National Institute of Standards and Technology scientist likened conductors to antennas that can arc to the oven wall or other metal, per [scientificamerican.com](https://www.scientificamerican.com/article/why-do-metal-objects-crea/). [3] [2] [6]

Shape matters more than the metal itself. Scientific American points out that the oven's own walls are metal, and that flat metal trays in some pizza packaging get hot and brown food without sparking. Sharp points and the many tiny edges of aluminum foil concentrate charge and cause a corona discharge. Slepkov said flat foil might not spark, but crumpled foil will quickly. Once a spark ionizes the air, it absorbs microwaves even more strongly than water does, so the spark can grow. Sparks can also occur without metal, as MIT's Caroline Ross showed with cut hot dogs. Sparking can damage the oven, though food is generally fine to eat afterward, according to Live Science. [1] [2] [5]