HomeFAQsWhy can’t you put metal in a microwave?

Why can’t you put metal in a microwave?

Dear Sparking Chef,

When you look in a mirror, your image is bouncing off of metal. The back of a mirror is coated with aluminum. (Older mirrors were backed with silver.) Not only do metals conduct electricity, like when electricity runs through wires to lights and video games; they also reflect electrical waves that hit them from outside their surfaces. It may seem a little bit complicated, but I’d bet you’ve already seen it. Electricity runs through wires, and the wires are shiny. They reflect light. It’s the way of the Universe.

The energy of light, heat, x-rays, radar, and in a microwave oven is what we call “electromagnetic” energy. It’s electrical and magnetic at the same time. We can think of this energy as moving like a ray of light. We say it’s “radiant.” It “radiates.” So, microwaves are “electromagnetic radiation.” So is sunlight and heat and radar and television signals. They’re all forms of the same thing. We have discovered that we can think of this electromagnetic radiation as traveling in waves. When the waves are short, they bounce off of metals, like light off of your mirror. When the waves are longer, they transfer energy into the metal, like the waves hitting a radio antenna on a car. Radio and television waves bounce off of you and trees and buildings and cars. Light waves are few tenths of a millionth of a meter long. Radio waves are a few meters long. That’s quite a difference. Well, microwaves are somewhere in between. They’re about 10 centimeters long, about as long your hand or a watch band unhooked. These waves are in between in how they behave around metals, too.

Microwaves happen to not pass right through water. Nor are they completely reflected by water. So, when we put something with water in it in the path of microwaves, say a potato or popcorn kernel, the water molecules start tumbling and jostling like crazy. The energy of the rubbing water molecules turns to heat. Your potato or popcorn heats up. When these waves hit metal like the aluminum foil or gold paint, the metal absorbs the microwaves like an antenna. The energy doesn’t turn to heat, roughly because metal is not water. (You know that, too.) The microwave energy has to find a place to go. It usually forms sparks that jump from air molecule to air molecule all the way back to the metal sides or bottom of the oven. Where the sparks comes and goes from the aluminum foil or metal paint, it gets hot. The energy is concentrated. It often burns a tiny hole or pit in the metal. It’s energy just looking for a place to spread out. It can’t; so, don’t put metal in a microwave. It’s a bit like people who travel in waves at the speed of light. Wait maybe microwaves aren’t like people. Oh well, they’re part of our world just like you.


Bill Nye

  • Frank H.T. Rhodes Class of ’56 Professorship

Space Sciences, Cornell University
Research Area:
Bicycling, baseball and science

Question From

Jeffrey Barken
Ithaca High School
Mrs. Lee
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