Microwaves heat food by emitting electromagnetic waves at a specific frequency, typically around 2.45 gigahertz. These waves are absorbed by water, fats, and certain other molecules present in the food, causing them to vibrate and generate heat. This process is known as dielectric heating.
However, when metal objects are placed in a microwave, a different phenomenon occurs. Metals reflect and conduct electromagnetic waves, including microwaves. When the microwaves encounter a metal object, such as aluminum foil or metal utensils, they induce an electric current in the metal. This current flows through the metal and can create sparks.
The reason why sparks are generated in the presence of metal in a microwave is that the high electric field created by the microwaves causes electrons to move rapidly within the metal object. If these electrons encounter a gap or a sharp edge, such as a pointy part of a fork or a crease in aluminum foil, they can ionize the surrounding air molecules, leading to a visible spark.
To prevent sparks and potential damage to the microwave or fire hazards, it is generally advised not to put metal objects inside a microwave. It's important to use microwave-safe containers and utensils made of materials that do not conduct electricity, such as glass, ceramic, or certain types of plastic specifically labeled as microwave-safe.