Disclaimer: Microwaves are nowhere near micron-sized, the size of a cell, or the thickness of your hair. With an operating frequency of ~2.5GH, the wave has a “size” of ~5 inches. [You can actually observe this by heating a chocolate bar in a non-rotating microwave and measuring the distance between squishy spots. Not as cool as microwaving split grapes, but it still proves a point.] This size is the reason why you can still see your food cook through small holes without the radiation escaping; they won’t fit through the holes in the metal and just reflect back into the chamber.
Microwave ovens heat food by exciting microwaves (a form of electromagnetic radiation) within a metal chamber. The molecules in the food, mostly water, absorb the radiation causing them to rotate and bounce into nearby particles. The resulting recoils induce particle movement and vibrations that are perceived as heat.
Note: The term “radiation” has been given a bad rap due to its relation to nuclear reactions and its “unconventional” applications. Radiation is actually an extremely broad term used to describe a lot of things. Atoms without electrons (Alpha particles), electrons without atoms (Beta particles), and electromagnetic waves (photons, including Gamma “particles”). All three can be detected at varying levels of energy and used in beneficial ways.
So what actually creates the microwaves? The most common device in ovens is the cavity magnetron, a cylindrical metal device with internal cavities that run parallel to the electron flow. The internal air is removed to create a vacuum, so the electrons can freely flow within the chamber without bouncing into gas molecules (N2,O2, CO2, CH4, engineer body odor, etc.) for efficiency. Additionally, magnets are placed on the magnetron to create a strong field that flows through the device (perpendicular to the cross-section shown below).
Electrons flow from negative to positive terminals in the magnetron. However, there is a vacuum in between the negative and positive terminals, requiring the electrons to escape the metal to complete the circuit. This is assisted through high voltage drive and heating the electron source (basically lots of energy!)
This is where ElectroMAGnetIC interaction (EMagic) converts electron flow into radiation. The electrons do want to make a straight beeline towards to exterior (+) of the magnetron, but the magnetic field tugs at the electrons sideways [the Lorentz Force]. As the electron changes direction, that magnetic tug constantly pulls it sideways resulting a circular path until it finally hits the exterior (see below). A lot of electrons undergo this, resulting in an “electron cloud” of energy.
Passing by the cavities, the electrons interact with the electrons that are currently in the exterior (electron drain) in the cavities, transferring energy to them causing them to “vibrate.” Unlike mechanical vibrations in a bell, the electrons flow and give off electromagnetic radiation. The size of the cavities in the magnetron, specifically the cavity’s circumference, is designed to excite the microwaves of the specific frequency for maximum water absorption (~2.5 GHz).
Most magnetrons are >50% efficient, at least in terms of (power out)/(power in). While this value doesn’t look too impressive, it’s actually really good based on the EMagic involved. In comparison, look at how solar panels work and how they can have a hard time even achieving >20% efficiency.
The radiation emitted also has quite a bit of noise, but is still acceptable for basic food applications. Just don’t expect cavity magnetrons to be utilized in radar to detect and characterize incoming (possibly enemy) aircraft.
Finally, the created radiation then leaves the magnetron, passes through a “window,” and bounces around the metallic chamber where you put your food. The window can be a variety of materials, including plastic and cardboard (I’ve seen both). It usually is opaque to us because it acts optically different when viewed with light (visible EM radiation, 350-700 nm). However, the material is transparent to microwaves, acting as a window (Assuming it’s not covered in wet spaghetti sauce. Keep your microwave clean before the mold gets to it!)
A lot more does happen, but this simplified explanation is a good start. It’s like an ice-breaker when meeting new people; an exciting conversation can lead to a new friend. [Insert your own connection here, my readers….]