How Does a Microwave Heat Food? (And Why It Doesn’t Cook From the Inside)

A microwave oven heats food without a flame or a hot plate. It uses radio-like waves that make molecules in the food move.

Spinning water

A household microwave produces waves at about 2.45 gigahertz. Water molecules are slightly positive at one end and negative at the other. The oscillating field tries to twist them back and forth billions of times per second; as they jostle against their neighbours, that motion becomes heat. Fats and sugars heat this way too, to a lesser extent.

It doesn’t cook from the inside

A common myth. Microwaves penetrate only about one to a few centimetres into typical food. The outer layer heats directly; the centre warms by ordinary conduction from the outside in. That is why thick food needs standing time.

Cold spots

Inside the oven, the waves form a pattern of strong and weak spots. The turntable moves food through them, but dense or frozen parts still heat unevenly — so stir halfway, cover the dish and let it stand.

Why ice heats slowly

In ice, water molecules are locked in a crystal and can’t rotate easily, so frozen food absorbs microwaves poorly — until a bit melts, which then heats quickly. That is why defrost settings pulse on and off.

The chocolate bar

Around 1945, Percy Spencer, an engineer working on radar equipment at the US company Raytheon, noticed a chocolate bar in his pocket had melted near a radar tube. Experiments with popcorn followed, and within a couple of years the first (very large) microwave ovens were on sale.