Final answer:
Microwave ovens heat food efficiently by emitting microwaves at a frequency that causes water molecules within food to rotate more quickly, transferring energy as heat to the food. The typical frequency used is around 2.45 GHz, which is specifically chosen to excite the rotational energy levels of water molecules.
Step-by-step explanation:
The efficiency of a microwave oven in heating food items containing water molecules is determined by the interaction between the microwave photons and the water molecules in the food. Microwave ovens use a specific frequency of microwaves to cause water molecules within food to rotate faster, a process known as dielectric heating. Most kitchen microwaves operate at around 2.45 GHz, which is suited to excite the rotational states of water molecules.
These microwave photons are absorbed by water molecules, causing an increase in their rotational energies. This added energy is transformed into heat, which is then transferred to the surrounding food, causing it to warm up rapidly. Microwave ovens are designed to target the rotational energy levels of water molecules, making them particularly effective at heating foods that contain water.
The energy of one photon of these microwaves at the frequency of about 3 × 10⁹ Hz (which is typical for microwave ovens) can be calculated using the Planck-Einstein relation. The relation states that energy (E) of a photon is equal to the frequency (ν) of the radiation times Planck's constant (h), or E = hν.