Why can't you put metal in a microwave? - Aaron Slepkov

American engineer Percy Spencer developed World War II RADAR technology

that helped detect Nazi airplanes—

but it would soon have other surprising applications.

One day in 1945, Spencer was standing near a RADAR instrument

called a magnetron,

a device that produced high-intensity microwaves that could reflect off planes.

Suddenly, he noticed that the candy bar in his pocket had melted.

He exposed other things to the magnetron and, sure enough,

popcorn kernels popped,

and an egg—well— exploded onto a colleague.

Soon after, the first microwave oven became available,

operating using the very same technology.

So, how does it work?

All light energy travels in waves of oscillating electric and magnetic fields.

These oscillations span a range of frequencies

comprising the electromagnetic spectrum.

The higher the frequency, the more energetic.

Gamma rays and X-rays have the highest frequencies;

microwaves and radio waves, the lowest.

Generally, light’s oscillating electric field exerts forces on charged particles,

like the electrons in a molecule.

When light encounters polar molecules, like water,

it can make them rotate,

as their positive and negative regions are pushed and pulled in different directions.

The frequency the light is traveling at

also determines how it interacts with matter.

Microwaves interact strongly with the water molecules found in most foods.

Essentially, they make the molecules jostle against each other,

creating frictional heat.

Household microwave ovens are fitted with cavity magnetrons.

When you activate a microwave oven,

a heated element within the magnetron ejects electrons,

and a strong magnet forces them to spiral outwards.

As they pass over the magnetron’s metallic cavities,

the electrons induce an oscillating charge,

generating a continuous stream of electromagnetic microwaves.

A metal pipe directs the microwaves into the main food compartment,

where they bounce off the metal walls

and penetrate a few centimeters into the food inside.