Why is it not safe to put metal vessel in microwave oven?

We all have heard that one should not put metal inside a microwave oven if one needs to use the appliance for a long time. Some may have had the firsthand experience of the behavior of the oven when they accidentally put a fork or spoon in it.
Contrary to popular belief, it is not unsafe to put all metals in microwave ovens. People often put metal inside the device not thinking about it. The hot pockets is a perfect example of that. It does have a thin aluminum foil around it, which absorbs the microwaves and helps to cook it properly, though leaving it for too long would result in it catching fire. And the inside walls of the oven are made of metal itself. So why can’t we put a metal plate or something in the oven?
To get an answer, we first need to understand the working principle of the oven. They work by electromagnetic radiation. A device called Magnetron is used to produce high frequency microwaves from AC current. These waves vibrate inside the metal walls and get absorbed by the water content in the food. This makes the organic molecules in the food oscillate as well, creating the heat necessary to cook the food.
When a metal object is placed inside the oven, instead of absorbing, it would reflect the waves and they would bounce off the metal walls of the oven, which would result in an erratic oscillation of the waves and a lot of heat, ultimately damaging the device. Other way the metal can damage oven is when it has sharp ends, like fork. Since metals are conductive, electrons will accumulate at the edges and would create an electric arc between the metal and the magnetron, damaging the device. Now you know where those mini lightning bolts came from the last time you forgot to take the fork out.
However, now there are metal containers and plates specifically designed for using in microwave oven. Recently, scientists have deemed it safe to put metal in microwave oven as long as its mass doesn’t exceed 30% of the total mass and there is only one piece of metal, and of course with no pointy bits. Even then, it is generally unsafe to put metals in the oven for its longevity.

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