Is it safe to put metallic objects into a microwave oven?

Many of us have probably been advised since young to not place metals, in any shape or form, into a microwave oven, unless we wish to warm up the house, in addition to heating up our food. But if we were careless and maybe stuck a metal spoon or fork or a piece of aluminium foil into our food before placing it into the microwave oven, would it blow up?

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Microwave ovens work by concentrating electromagnetic waves, of the microwave frequency range of the electromagnetic spectrum, in the cavity of the oven. After absorbing the microwave radiation, the water molecules in the food that is placed in this cavity begin to vibrate, gaining kinetic energy and they would thus heat up. Based on this working principle, microwave ovens are able to heat up food that contains moisture. 

What happens when these electromagnetic waves hit metal surfaces? Being poor absorbers of microwave radiation, most of the waves are actually reflected off the metal surface. However, some are absorbed and these waves would induce a small current within the metal. In fact, metals are such good reflectors of microwave radiation that the interior walls of the microwave oven is actually made of mostly metal. This design feature helps to concentrate the microwave radiation produced by the magnetron within the cavity of the oven.

If a metallic object is placed within the cavity of the microwave, there would be a concentration of microwave radiation between the object and the interior walls of the oven, producing a highly concentrated electric field in the cavity of the oven. A powerful electric field with a very high potential difference between the walls of the oven and the metallic object will be produced. This high p.d. could cause the breakdown of air within the oven and sparks would thus be created between the object and the oven walls. These sparks would damage the internal circuitry of the oven and may even become a fire hazard. 

Sounds very dangerous? But here’s the catch… Metallic objects without sharp edges or pointed tips are generally safe to be placed in the microwave oven. A metal bowl or flat metal tray would be safe in the oven. That is to say, sparks would likely not be observed. However, a metal fork or a crumpled piece of aluminium foil may not be as safe. The reason is that the surface charge density at such locations is higher and thus, electric fields at these locations would be much more concentrated, allowing higher potential differences to be generated and the breakdown of the air becomes more likely to occur.