Microwaves are effective at heating food because of their interaction with water molecules. Microwaves are a form of electromagnetic radiation that emits energy in the microwave frequency range. When microwaves pass through food, they are absorbed by water molecules present in the food.
Water molecules have a unique molecular structure with a positive end and a negative end, making them polar molecules. The electric field of the microwaves causes these water molecules to rotate rapidly, and this rotational movement generates heat. As a result, the food containing water molecules heats up as the water molecules rapidly absorb the microwave energy and transfer it as heat to the surrounding food particles.
On the other hand, most plates and containers used in microwaves are made of materials like glass, ceramic, or plastic, which do not contain many water molecules and are not polar like water. Therefore, these materials do not effectively interact with microwaves, and as a result, they do not absorb much microwave energy or get heated directly.
However, it's essential to note that while plates themselves may not heat up significantly in the microwave, the food placed on them can transfer heat to the plates through conduction. This happens when the hot food comes into contact with the plate, and the heat is transferred from the food to the plate surface. That's why after microwaving food, the plate or container holding the hot food can become warm due to this process.