I put a 1/4 inch thick clear glass container into the microwave with a plate on top and put it in for almost 5 minutes (there was lots of soup). When It came out the soup was really hot but I could still hold the top parts of the container. After measuring the soup's heat it was about 120-130 degrees but the top part felt cool to the touch. Why didn't it heat up like the soup did?
The microwaves are primarily designed to vibrate/heat the water molecules in the food, as a way of ensuring that the foods gets cooked evenly. An aid to this process is the rotating plate within the machine.
Microwaves that cook your food pass through plastics, glass, and ceramics, with mimimal heating, as their water content is low and they are less prone to heating, explaining why you can pop your (almost ready to go) chicken curry and rice, along with its plastic packaging, straight into the microwave. It is also this feature of microwaves that makes them so energy efficient; they heat only the food and nothing more.
However, don't try to put eggs in a microwave, they will become minibombs as the water heats up, turns to steam and then blows the eggshell apart. Cups of water are not recommended, nor is just pressing the start button without food or liquid to absorb the microwaves, as the magnetron (which is what cooks your food), ends up absorbing the microwaves instead, which can damage your microwave and may even start a fire.
Metals, on the other hand, reflect these radio waves, a characteristic very cleverly put to use in the walls of the microwave such that no waves escape and cook anyone in the kitchen! However, you can see sparks from the edge of some decorative ceramic plates appearing now and again, from the microwave radiation.
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