Microwaves with a frequency too high to be heard by humans can be used to heat food due to the specific interactions they have with water molecules and certain other substances.
Microwaves are a form of electromagnetic radiation, and their high frequency (around 2.45 GHz for household microwaves) corresponds to wavelengths in the microwave range. These microwaves are specifically designed to interact with water molecules, which are present in most food items.
Water molecules have a dipolar nature, meaning they have a positive end (hydrogen) and a negative end (oxygen). When exposed to microwave radiation, the electric field component of the microwaves causes the water molecules to rapidly align themselves with the changing electric field. This alignment causes the water molecules to rotate, generating molecular friction and generating heat in the process. This process is known as dielectric heating.
The rapid rotation and friction among the water molecules throughout the food result in the transfer of heat energy, which evenly and quickly heats the food. This is why microwaves are efficient at heating food, as they directly target the water molecules within the food rather than heating the entire oven or the surrounding air.
It's important to note that not all substances respond to microwaves in the same way. While water molecules efficiently absorb microwaves, other substances like fats, sugars, and some plastics can also absorb microwave energy to a certain extent. However, substances like glass, ceramics, and certain plastics are transparent to microwaves and do not significantly absorb them, which is why microwave-safe containers are often made of these materials.
Overall, the ability of microwaves to heat food is based on their specific interaction with water molecules and other substances that absorb microwave energy, allowing for efficient and controlled heating.