A microwave oven floods its cavity with radio waves at about 2.45 gigahertz. Water molecules are electrically lopsided, with a slightly positive end and a slightly negative end, so they try to align with the wave's electric field. The field reverses direction billions of times a second, and the molecules flip back and forth trying to keep up.
All that flipping means molecules jostling their neighbours. Jostling molecules is what heat is. The food heats itself, from the inside of its own molecules, using energy the oven delivered.
Where microwaves sit on the spectrum
Microwaves are the same kind of thing as visible light and radio: electromagnetic radiation, differing only in wavelength. The spectrum is one continuous ruler.
| Type | Roughly | What it does to matter |
|---|---|---|
| Radio | Metres | Carries signals |
| Microwave | Centimetres | Rotates water molecules |
| Infrared | Micrometres | Felt as radiant heat |
| Visible light | Hundreds of nanometres | Scattered by air, seen by eyes |
| Ultraviolet | Tens of nanometres | Enough energy to damage DNA |
That last row is the boundary that matters. Below ultraviolet, radiation is non-ionizing: it can shake molecules but cannot knock electrons off them or break chemical bonds. Microwaves sit well below it, with roughly a millionth of the energy per photon that ultraviolet carries. It is the same continuum on which blue light gets scattered by the atmosphere, just far along the other end.
Why 2.45 GHz
A common claim is that this frequency is the resonant frequency of water. It is not, and the real reason is more mundane.
Water absorbs microwaves across a broad range. 2.45 GHz was chosen partly because it is in a band set aside internationally for industrial and domestic use rather than communications, and partly because it penetrates food to a useful depth. A frequency that water absorbed too strongly would deposit all its energy in the outer few millimetres and leave the middle cold.
Why food heats unevenly
Three reasons, all of them things you have noticed.
Standing waves. Microwaves bounce off the metal walls and interfere with themselves, producing a fixed pattern of hot and cold spots. This is why there is a turntable: not to cook evenly by magic, but to drag the food through the pattern. You can map the pattern by microwaving a tray of marshmallows with the turntable removed and looking at which ones melt.
Penetration depth. Microwaves lose strength as they go in, and reach only about a centimetre or two before most of their energy is gone. Beyond that, the middle of a large item heats by ordinary conduction from the outside, exactly like a conventional oven. The "cooks from the inside out" claim is simply false.
Water content varies. The energy goes where the water is. A dense wet centre in a dry pastry becomes far hotter than the pastry, which is the mechanism behind every burnt mouth from a jam doughnut.
The standing pattern, the shallow penetration and the uneven water are why standing time is part of the instructions. Heat needs a minute to conduct into the parts the microwaves never reached.
Metal, and why sometimes it is fine
Microwaves induce currents in metal. In a thin or pointed piece the charge concentrates at the edges, the voltage climbs, and the air breaks down into a spark. Fork tines, foil edges and metallic paint on a plate rim all do this.
Smooth, thick, rounded metal simply reflects, which is why the walls and the door mesh are metal. The mesh holes are a few millimetres wide and microwaves are about 12 centimetres long, far too big to fit through, which is why you can see in and the radiation cannot get out.
Is it dangerous
Not in the way the word radiation suggests. Microwave radiation is non-ionizing, so it cannot alter DNA and cannot make food radioactive. Food from a microwave is heated food and nothing else.
It can burn you thermally, which is a different hazard. Superheated water is the sharpest version: clean water in a smooth cup can be heated past boiling without bubbling, then erupt when disturbed. Ovens are also required to leak below strict limits, and a door that no longer seals is a genuine reason to replace one.
On nutrition, microwaving is generally gentler than boiling, because it is fast and uses little water. Most nutrient loss in cooking is heat and leaching, and this method is comparatively low on both.
For reheating a single portion it also uses much less energy than heating a whole oven, which is a small but real item on a household carbon footprint.
The short answer
A microwave makes the water in your food vibrate, and the friction is the heat. It reaches only a centimetre or two, so the rest cooks by conduction, and the cold middle is geometry rather than a fault. It is not ionizing, it does not irradiate your dinner, and the only real hazard it presents is that things get hot.