How Things Work: Microwaves

Molecules in food absorb energy from microwaves, generating heat. (credit: Jesse Kummer/Personnel Manager) Molecules in food absorb energy from microwaves, generating heat. (credit: Jesse Kummer/Personnel Manager)

We love the microwave. It’s the hunger-squashing, late-night savior: Just pop in some food, and any appetite will be satisfied. It also saves time and effort, eliminating oven preheat time or any need to adjust gas settings. The microwave is a reflection of today’s busy world: instant gratification.

In any room where food is made, from designer kitchens to a simple dorm room, a microwave can be found. But just how do they work? How can they turn yesterday’s leftovers into today’s dinner with just the touch of a button?

The science behind microwaves comes from its namesake radiation. Microwaves are a form of electromagnetic radiation, which are waves of energy that propagate through all space. Microwaves are created inside a microwave oven via a device called a magnetron, which creates a flow of electrons inside a vacuum using a magnetic field.

Visible light, which reflects off objects so our eyes can detect it, is also a form of electromagnetic radiation. Ultraviolet light, which causes sunburns, is also electromagnetic waves. However, visible light, ultraviolet light, and other common forms of electromagnetic waves have a higher energy than microwaves. So why doesn’t our food spontaneously cook all of the time?

The answer lies in a specific property of microwave radiation. Microwave ovens usually use waves with a frequency of 2.5 gigahertz, according to an article on When the microwaves come into contact with water molecules, fat molecules, and sugars — which are common components of many foods — the energy is absorbed by these molecules. Upon this absorption, the molecules begin to vibrate quickly, creating friction; this friction creates heat. This is the same phenomenon that occurs by rubbing palms together.

Microwaves can cook food more evenly than a conventional oven because the radiation can excite molecules from anywhere within the food, while the heat from conventional ovens must permeate from the outside to the inside of the food. In addition, the food is the only object in the microwave that heats; the rest of the microwave, including the air, remains cool.

Electromagnetic radiation can be dangerous — the Kevlar that protects a patient receiving an X-ray is worn for a reason. However, microwave oven doors are designed to contain the microwaves. As explains, the doors are composed of a metal mesh that has holes that are smaller than the wavelength of microwaves. This works effectively as a metal wall between the inside and outside of a microwave oven and is why we can safely see our food cooking.

The United States Food and Drug Administration created a standard for the amount of microwave radiation that can escape from a microwave oven, which is five milliwatts of microwave radiation per square centimeter at approximately two inches from the oven surface. This level is below that which is known to harm humans.

Even though microwave radiation seems innocuous, there are some safety hazards associated with them. A well-known phenomenon is the sparking of metals inside a microwave oven. This happens when the metal has some pointed surface, as in the case with forks, aluminum foil, and takeout container handles. The metal will resonate with the microwave radiation, causing a high voltage to build up; since air can conduct electricity, a spark forms. However, this can easily be avoided by removing metal from the food to be microwaved. Other hazards include exploding containers due to the buildup of steam from heating food, which can be prevented by removing lids or creating gaps through which the steam can escape. In addition, cooking nothing in a microwave is harmful to the machinery, as it causes microwaves to accumulate inside the cooking chamber. The magnetron, which is used to produce the microwaves, will eventually absorb the radiation and be destroyed.

When used correctly, microwaves are safe, save time, and can give us the warm food we want when we want it.