The microwave oven is one of the great inventions of the
20th century - millions of homes in America have one.
Microwave ovens are popular because they cook food incredibly
quickly. They are also extremely efficient in their use of
electricity because a microwave oven heats only the
food - nothing else.
A microwave oven uses microwaves to heat food.
Microwaves are radio
waves. In the case of microwave ovens, the commonly used
radio wave frequency is roughly 2,500 megahertz (2.5
gigahertz). Radio waves in this frequency range have an
interesting property: they are absorbed by water, fats and
sugars. When they are absorbed they are converted directly
into atomic motion - heat. Microwaves in this frequency range
have another interesting property: they are not absorbed by
most plastics, glass or ceramics. Metal reflects microwaves,
which is why metal pans do not work well in a microwave oven.
How Microwave Ovens Cook
You often hear that microwave ovens cook food
"From the inside out." What does that mean? Here's an
explanation to help make sense of microwave cooking.
Let's say you want to bake a cake in a conventional oven.
Normally you would bake a cake at 350 degrees F or so, but
let's say you accidentally set the oven at 600 degrees instead
of 350. What is going to happen is that the outside of the
cake will burn before the inside even gets warm. In a
conventional oven, the heat has to migrate (by conduction)
from the outside of the food toward the middle (See the HSW
article entitled How a Thermos
Works for a good explanation of conduction and other heat
transfer processes). You also have dry, hot air on the outside
of the food evaporating moisture. So the outside can be crispy
and brown (e.g. - bread forms a crust) while the inside is
In microwave cooking, the radio waves penetrate the food
and excite water and fat molecules pretty much evenly
throughout the food. There is no "heat having to migrate
toward the interior by conduction". There is heat everywhere
all at once because the molecules are all excited together.
There are limits of course. Radio waves penetrate unevenly in
thick pieces of food (they don't make it all the way to the
middle), and there are also "hot spots" caused by wave
interference, but you get the idea. The whole heating process
is different because you are "exciting atoms" rather than
In a microwave oven, the air in the oven is at room
temperture, so there is no way to form a crust. That is why
foods like "Hot Pockets" come with a little cardboard/foil
sleeve. You put the food in the sleeve and then microwave it.
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.
Microwave ovens are
described by several interesting links on the web. Try these:
- For an excellent discussion of the different parts of a
microwave oven and how they work together, click
- For a great collection of in-depth questions and answers
about microwaves, click