It is one of the most familiar machines in the kitchen. You put in a cold plate of leftovers, press a button and a couple of minutes later your food is hot. But what is actually going on behind that little glass door? The answer is a neat bit of everyday physics, and understanding it helps you heat food more evenly.

Microwaves are a kind of light

Microwaves are a type of electromagnetic wave, the same broad family that includes radio waves, visible light and X rays. What makes each type different is its wavelength. Microwaves sit between radio waves and infrared light. You cannot see them, but they carry energy.

Inside a microwave oven, a part called a magnetron produces these waves. They bounce around the metal walls of the oven and pass into the food.

Why the food gets hot

Food contains a lot of water. Water molecules have a slightly positive end and a slightly negative end, a bit like tiny magnets. Microwaves create an electric field that flips back and forth extremely quickly. The water molecules try to line up with this changing field, so they twist and jiggle constantly.

All that movement makes the molecules bump into their neighbours, and that energy shows up as heat. Fats and sugars in food can also absorb microwave energy, which is why some foods heat faster than others.

This is also why a plate or bowl may stay fairly cool at first. Many containers contain little water, so the microwaves pass through them without much effect. The container usually warms up later, because the hot food heats it.

Does it cook from the inside out?

A common belief is that microwaves cook food from the inside out. That is not quite right. Microwaves mainly heat the outer layer of food, typically the outer few centimetres. Heat then travels further inward by conduction, the same slow process that happens in a regular oven.

This is why a thick piece of food can be hot at the edges and still cold in the middle.

Why heating is uneven

You have probably noticed hot spots and cold spots. There are a few reasons:

  • Standing waves. Microwaves bouncing around the oven overlap. In some places they reinforce each other and in others they cancel out, creating hotter and cooler zones.
  • Different ingredients. Parts of a meal with more water or fat may heat faster than drier parts.
  • Shape and thickness. Thin edges heat quickly while thick centres take longer.

The turntable in many microwaves helps by moving food through the hot and cool zones. Some ovens use a rotating part hidden under the floor or in the ceiling to spread the waves instead.

Why standing time matters

Many food packages tell you to leave food to stand for a minute or two after heating. This is not just a suggestion. During standing time, heat keeps spreading from the hotter areas to the cooler ones, so the temperature evens out.

For food safety, it is important that food is heated thoroughly all the way through. Stirring halfway, covering food loosely and allowing standing time all help. Follow the instructions on packaging where they are given.

What about metal?

You may have seen sparks when metal goes into a microwave. Thin or pointed metal, such as foil edges or the gold trim on a plate, can concentrate electric charge and cause sparks. That can damage the oven or the dish.

The metal walls of the oven itself are designed to contain and reflect microwaves. The mesh in the door works the same way. Its holes are small enough that the microwaves stay inside, while visible light, which has a much shorter wavelength, passes through so you can watch your food.

Always check that containers are labelled as microwave safe. Some plastics are not suitable for heating, and some ceramics have metallic glazes.

Tips for better microwave heating

  • Spread food out in an even layer rather than piling it up.
  • Put thicker parts towards the outside of the plate.
  • Cover food loosely to trap steam and help it heat evenly.
  • Stir or turn food halfway through.
  • Let it stand before eating.
  • Check that it is hot all the way through.

The takeaway

A microwave oven heats food by making its molecules, especially water, move rapidly. That motion turns into heat, mainly in the outer layers, which then spreads inward. Once you know that, the advice to stir, spread food out and wait a minute makes perfect sense.