Microwaves work by emitting electromagnetic waves, specifically microwaves, which have a specific frequency that causes water molecules in food to vibrate. These vibrations generate heat, which in turn heats up the food.
The reason why the food gets heated up more than the container is due to the nature of the materials involved. Microwaves are absorbed by substances containing water molecules, such as food, but they are not readily absorbed by most plastics, glass, ceramics, and other materials commonly used for microwave-safe containers.
When microwaves encounter the water molecules in the food, they transfer energy to them, causing them to vibrate rapidly and generate heat. This process is known as dielectric heating. The heat generated within the food then transfers to the surrounding parts, gradually heating up the entire item.
On the other hand, materials like plastic, glass, and ceramics are transparent to microwaves, meaning that they allow the waves to pass through them with minimal absorption. As a result, the container remains relatively cool during microwave heating because it doesn't directly absorb a significant amount of microwave energy.
However, it's important to note that not all materials are microwave-safe. Some containers, especially those made of metal or lined with metal, should not be used in the microwave because they can reflect or concentrate microwaves, causing sparking or even fire hazards. Always check the manufacturer's instructions to ensure that a container is safe for use in the microwave.