TIL: WHY You Shouldn't Put Metal In The Microwave

We've ALL been taught that you shouldn't put metal in the microwave.
Maybe you've even seen some of those videos where a microwave caught on fire, or saw it happen live, when someone did accidently turn on the microwave when something containing metal was inside.
But what exactly causes this reaction?

How a Microwave works

The most important part of a microwave is the Magnetron - a vacuum tube with a lot of power, that generates a specific type of electromagnetic radiation: the so-called microwaves.
These microwaves are then being led into the main chamber, where they then get absorbed by the water, fat and sugar molecules in your food and heat it up.
The walls of the chamber are actually made up of thick metal, so when the microwaves strike them, the electrons in the wall are just being vibrated a little, but the wave just bounces back off.
Also, a metal mesh is attached to the door of the microwave: the holes in the mesh are smaller than the wavelength of the microwaves, so they waves can't go through.
All of this ensures that your food is being heated up, while the microwaves themselves stay inside the chamber.

Metals in the Microwave

So it seems like there is quite a lot of metal that makes up the components of a microwave already, but why can't you put any inside then?
You CAN put certain types of metal in the microwave - but it all depends on their shape and thickness.
Metals conduct a lot of electricity, and there are lots of freely moving electrons inside of them.
When a metal is very thin, for example foil, it can't withstand all the electron activity that is being created when the microwaves strike with the metal. It heats up extremely quickly and can damage your microwave.

The shape of the metal also plays a big factor here.
Maybe you've already accidently had a metal spoon in the microwave, and nothing bad happened?! That's quite possible.
When something has curves or sharp edges, the electromagnetic field concentrates along those areas - the electrons are accumulating there.
So then together with the microwaves, the air around that metal objects gets ionized through the buildup of charged particles. This is also what creates that popping noise.
And if this charge buildup gets too high, there can even be an arch of electricity shooting out to the nearest metal object - which is usually the wall of the microwave.

So this is the reason why you should never put a fork in the microwave, but you might get lucky if you've forgotten the spoon - a spoon has no sharp edges where the electrons could concentrate, it spreads the charge instead of causing a buildup.

Metals in packaging?

Maybe you've noticed that certain foods that are specifically designed to be heated in the microwave (for example hot pockets, or many other microwaveable snacks) seem to be packaged with a layer of metal.
And that is correct: the packaging has a thin metallic layer, which is then again coated with layer of non-metallic material on the outside - this also called the crisping sleeve.
This has a specific reason: the metal layer inside heats up the food even more quickly and helps create a crispy crust, but the isolation from outside prevents possible dangers.
But still, if you leave these foods in the microwave for too long, the packaging could still start to burn!