Why does microwave food heat unevenly?

You reheat a bowl of leftovers and take a bite. It is scalding. The next bite, about an inch away, is still cold. The oven ran the whole time, so where did the heat go? Inside the oven, the energy gathers in bands that stay in one place.

Why does the heat gather in bands?

A microwave oven fills a metal box with microwaves, a kind of radio wave. They bounce off the walls and run back over themselves. Where the waves line up, they add into strong patches. Where they meet out of step, they cancel into weak ones.

The patches do not drift. They stay fixed in the oven, like the still points on a plucked guitar string. Physicists call this pattern a standing wave. OpenStax: The Electromagnetic Spectrum.

Home ovens run at about 2.45 gigahertz, which makes each wave about 12 centimeters long. Strong patches repeat every half wave, so they sit about 6 centimeters apart. Food on a strong patch keeps heating. Food between patches gets very little. Wikipedia: Microwave oven.

▶ Play with this now in Microwave Hot Spots: turn the turntable on and watch the stripes blur into rings.

What does a turntable change?

Our model scores each bite against a spot that sits on a strong band the whole time. With the turntable off, a bite on a band scores nearly 100 percent after 12 seconds. A bite 3 centimeters farther out scores about 1 percent. The food prints stripes.

Now turn the turntable on. The bands stay put, but every bite rides a circle through them. After one full turn, the same two bites score about 60 and 43 percent. Across the whole dish, scores narrow from 0–100 percent to about 30–98 percent. The stripes blur into rings.

That is closer, but not even. One spot never moves at all: the center of the turntable. In our model it sits on a strong band, so it scores 100 percent either way. Real turntables can leave an uneven spot in the middle too. Wikipedia: Microwave oven.

Some ovens move the waves instead of the food. A spinning, fan-like stirrer, often hidden in the ceiling, bounces them in changing directions. Either way, the hot spots get spread around. OpenStax: College Physics.

What does the model leave out?

Our film and Microwave Hot Spots use a one-dimensional slice of the field. They show a relative energy score, not a temperature. Real ovens are messier:

• Three dimensions. A real oven holds a tangle of overlapping wave patterns. They depend on the oven, the food and where it sits.
• The food. Different parts of a meal absorb energy at different rates. Microwaves reach only about 1 centimeter into most foods. The middle of a thick dish warms as heat spreads in from outside. Wikipedia: Microwave oven.
• Spreading heat. Heat flows from hot bites to cold ones, during cooking and after. The model counts absorbed energy only, so it never moves heat around.

The frequency slider is there so you can watch the bands move. Home ovens do not let you change it. A turntable helps, but it does not guarantee even heating. That is why cooking instructions often say to stop and stir. This model is not food-safety guidance.

Can a chocolate bar show the hot spots?

You can map the bands with a large, flat bar of chocolate. This version follows a demo from the University of Arizona's Wyant College of Optical Sciences. These steps are a suggestion, so adapt them to the child beside you. An adult should run the oven.

First, check your oven's manual in case it says to keep the turntable in place. Then lift out the glass tray and the ring of wheels under it. Unwrap the chocolate, because foil does not belong in a microwave. Lay the bar flat on a large paper plate that will not turn. FDA: Microwave Ovens.

Predict. Ask your child what the chocolate will look like after a few seconds. Will it soften all over, from the edges, or in patches? If patches, how far apart will they be?

Try. Heat the bar for about 5 seconds at a time, and look between bursts. Stop as soon as soft, glossy spots appear, before anything scorches. Let it cool for a minute before anyone touches it. Then measure the gaps between the soft spots with a ruler.

Explain. The soft spots are the strong patches, and the chocolate sat still under them. In many ovens they land about 6 centimeters apart, half a wave. Yours may look patchy rather than striped, because a real oven is three-dimensional. Put the turntable back when you are done.

For a bonus step, double your gap to get one full wave. Multiply it by the oven's frequency, often printed on a label as 2450 MHz. A 12-centimeter wave gives about 294,000 kilometers per second. That is close to the speed of light. University of Arizona: Measure the Speed of Light with a Microwave.

OpenStax suggests a similar test with cups of water placed along a line. OpenStax: College Physics. For the next question, watch why two waves can cancel out. The same adding and canceling builds every hot band.

Microwave Hot Spots is an ExplainerTools Original.