How Do Microwave Ovens Work?

They’re convenient, ubiquitous and easy to use. But how do these things work, exactly? What do people mean when they say a microwave cooks food inside out?

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Episode Transcript

Speaker 1 (00:02):
Welcome to brain stuff from how stuff works. Hey, brain stuff.
This is Christian Sager. There is no denying it. Microwave
ovens are super convenient. They can heat food much more
quickly than a conventional oven, although not always with the
same results. And there's an entire industry of food made

(00:23):
specifically for these nifty gadgets. But how do they work? Yeah,
of course you're saying right now, well microwaves. Sure, yeah, microwaves.
But what the heck are microwaves? Good question, Christian. Microwaves
are a type of wave on the electro magnetic spectrum,
and their sandwiched between radio waves and infrared radiation. In

(00:44):
the case of microwave ovens, the most common wave frequency
is roughly two thousand, four hundred and fifty mega hurts.
That's about two point four or five giga hurts. Waves
in this frequency range have an interesting property. They're absorbed
by water, fats, and sugars, and once absorbed, they're converted
directly into atomic motion, which we like to use as heat.

(01:08):
These waves have another convenient property. They're not absorbed by
most plastics, glass, or ceramics. Metal, however, does reflect microwaves,
which is why it's a bad idea to leave a
spoon and you cheese dip when the oven's on. It's
also why the devices have metal walls for reflection. You'll
often hear people say microwave ovens cook from the inside out, right,

(01:32):
that's the key to the speed. Think about it like this.
Let's say you're baking a cake in a conventional oven.
Normally you would bake it at three and fifty degrees
fahrenheit or a hundred and seventy seven degrees celsius. But
this time you accidentally set the oven at six hundred
degrees fahrenheit. The outside of the cake will burn before

(01:53):
the inside even gets warm, and you'll have ruined somebody's birthday.
In a conventional oven, the heat has to grate by
conduction from the outside of the food toward the middle. Hot,
dry air on the outside evaporates moisture, so the outside
can be crispy and brown like the crust unbread while
the inside is moist In microwave cooking, the radio waves

(02:16):
penetrate the food and excite water and fat molecules more
or less evenly throughout. No heat has to migrate toward
the interior by conduction. There's heat everywhere all at once
because the molecules are all excited together. There are limits
to this, though. Microwaves penetrate unevenly in thick pieces of food.

(02:37):
They don't make it all the way to the middle,
and there are also hot spots that are caused by
wave interference. But you get the idea. The heating process
is different because you are exciting atoms rather than conducting heat.
Inside a microwave oven, the air is at room temperature,
so there's no way to form a crust. That's why

(02:58):
microwaveable pastries or hot pockets sometimes come with a little
sleeve made out of foil and cardboard. The sleeve reacts
to microwave energy by becoming very hot. This exterior heat
lets the crust become crispy, as it would in a
conventional oven. Check out the brainstuff channel on YouTube, and

(03:22):
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