It’s lunchtime, time to get your Lean Cuisine queued up. You unwrap it, then zap it. When you take it out of the microwave, it’s boiling hot on the edges and freezing cold in the middle. You know what I’m talking about.

And you wonder: why doesn’t it heat my food like a regular oven?

This is a question beyond the interest of science. It carries important quality-of-life-and-lunch consequences.

To answer it, let’s think about what it means to heat something. Heat is really just molecules moving around. When you get something hot, you get the molecules agitated.

Microwaves and regular ovens have somewhat different ways of agitating food molecules (though the way they agitate cooks may be similar). A regular oven, once you’ve preheated it, is full of hot air. When you put your tasty treat inside, the heated air interacts with the cooler surface of your food and moves some of the molecules around. Over time, the molecules exposed to the surface transfer their heat energy to the molecules next to them through a process called conduction. Eventually, the heat gets conducted all the way to the center of your meal, but it takes a while, which is why you have to leave your tasty treat in the oven for what seems like forever when you’re hungry.

Bread baking in a conventional oven
In this conventional oven, the baking bread is surrounded by hot air, which lends it its crispy crust.

In contrast, a microwave tickles your treat molecules with radio waves. There’s no hot air in the microwave, and it heats your food without heating anything else. If you think about it, this seems kind of weird and maybe a little magical. Here’s how it works: a hollow-barreled magnetic tube called a magnetron emits radio waves into the oven. These waves, with wavelengths of about 12 cm, bombard the water, fat, and sugar molecules in the food, and set them flip-flopping. A microwave oven can do this while using much less energy than the oven requires, and the radio waves quickly get the molecules in motion.

The catch, which also explains the bubbling-and-freezing-at-the-same-time phenomenon, is that the microwaves only penetrate about an inch or inch and a half into your tasty treat. To get the heat deeper than that, you’re relying on conduction, just like in your conventional oven. Or, as all microwave chefs know, giving your treat a stir helps shift the unheated molecules to that all-important outer inch where the action happens.

The inside of a magnetron from a microwave, minus its magnet.

The microwaves, which are part of the same electromagnetic spectrum as visible light and X-rays, are “tuned” to a frequency that works its magic on the relatively loosely ordered molecules in your food, but that doesn’t have much effect in more solid materials like ceramics and glass. But these waves don’t penetrate all parts of your tasty treat equally, either. Water molecules, for instance, are much more readily tossed about than fat.

You can demo this yourself: take two small, identical dishes. Put water in one and the same amount of oil in another. Heat them together in the oven for about 30-45 seconds. Which is warmer when the bell rings?

Microwaves are also notorious for simply heating unevenly: you might find a pocket of much hotter lunch in your otherwise lukewarm dish. This is because, like any electromagnetic waves, microwaves bounce off of reflective surfaces (like those of the oven walls) and concentrate more in some areas than others. This is where the turntable comes in handy: it moves different parts of your food into and out of the hot spots, distributing the heating effect evenly.

You can show yourself a faint glow of this effect by using your microwave to light up an incandescent bulb. If you try your hand at this, do it carefully, and only for three seconds at a time. You’ll see the bulb light up more brightly as it moves through the hot spots, and fade a bit as it goes through the “cooler” areas. You’ll find instructions and some explanation on the Oregonian website. Or, for a little safer show, watch this video from the Discovery Channel:

There’s plenty of other fun to be had with a microwave, and also quite a few clever and potentially hazardous experiments. For safety’s sake, I’m compelled to suggest you turn to YouTube rather than your own microwave for these. One of my favorites is this lovely slow-motion film of foods theatrically exploding in the microwave. You could try these at home, but be prepared for doing some cleanup.

Science in Your Life: The Magic Microwave 28 March,2012Robin Marks


Robin Marks

Robin Marks is the owner and operator of Discovery Street Tours, which offers science-themed walking tours in San Francisco. She is also a long-time science writer, and president of the Northern California Science Writers' Association. She loves to climb big hills, investigate tidbits of everyday life, and do chemistry experiments with her food.

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