In the modern kitchen, the microwave stands out as a compact wonder that saves time and money. Need to melt better? Put it in the microwave oven. Hankering for an afternoon snack? Pop a frozen pretzel in the microwave. You can have delicious food in seconds at a price that’s lower than what you’d pay at a fast food restaurant. Even though many people use this appliance daily, they may have no idea how it works. Take a closer look at the science behind how microwaves heat your food.
Making Molecules Dance
Your microwave oven gets its name from the kinds of electromagnetic waves it generates: microwaves. Approximately five inches long, the waves consist of electricity and magnetism, both of which move in an up-and-down pattern. Television and radio waves are similar to microwaves, but they’re much longer. In fact, the crests of these long waves may be several miles apart!
When set to a frequency of 2,450 megahertz, the microwaves can get absorbed by some common compounds found in food: fats, sugars, and water. When these organic compounds take in the microwaves, they convert them into tiny movements that happen at the atomic level. In essence, this molecular dance party is what creates the heat that warms the food.
While water, sugars, and fats absorb microwaves, many other common compounds do not. That’s why you can use containers made of sturdy plastic, ceramic, or glass in the microwave oven without fear that they’ll melt. It’s this selective absorption of microwaves that makes this appliance so practical. They’d hardly be useful at all if microwave ovens heated up everything compound you put in them.
Considering the Practical Implications of Microwaves
Contrary to conventional wisdom, microwaves do not cook food from the inside out. How the heat warms the food depends entirely on the food’s composition. If its insides consist primarily of water, as with a blueberry pie, then the water will heat up faster than other compounds, making it seem as if the food is cooking from the inside out. The pie could have a piping hot center and an outer crust that’s lukewarm. But when you heat up a dish in which the water is more evenly distributed, the microwaves tend to cook the food from the outside in.
The microwaves only impact about an inch and a half of the food. Thanks to the conductive properties of meat, bread, liquid, and vegetables, the heat moves through the food and warms up the inside.
Containing the Microwaves
There’s one other important consideration in investigating the functionality of this popular appliance: How are the microwaves contained within the appliance? Metal is the key. When it encounters microwaves, metal reflects it. So, manufacturers put metal in the walls of microwave ovens. Able to contain the radiation, the appliance’s metal walls prevent microwaves from escaping and impacting anyone or anything that happens to be near the microwave oven.
Because metal reflects microwaves, you can’t use a metal container in your appliance. It will reflect the radiation, causing it to bounce around wildly inside the appliance. With such erratic movement, the microwaves can damage the interior of the appliance, cause sparks, and even set food on fire.
In some special cases, thin metal sheets can be used safely in the microwave oven. In fact, you may have noticed that frozen pizzas and some other foods sit on a thin metal sheet. Frozen food manufacturers have figured out a way to safely harness the metal’s ability to reflect microwaves to get frozen pizza crusts crispy. Ultimately, the best practice is to avoid putting any metal, including aluminum foil, in the microwave.
Generating the Microwaves
To heat the food you place inside the microwave oven, the appliance has to produce the microwaves. The mechanism that creates microwaves is called a magnetron. Its most important components are oscillators that generate microwaves.
Originally, the magnetron was not intended to heat food. It was a tool for radar systems that helped airplanes and ships navigate through treacherous weather. By chance one day, an engineer discovered the magnetron’s ability to heat food. When a man named Percy Spencer turned on a magnetron one day, he happened to have a chocolate bar in his pocket. He soon discovered the magnetron generated enough heat to cause the bar to melt. In addition to making an unfortunate mess on his pants, the melted chocolate sparked an idea. He wondered if the magnetron could cook food.
Eventually, Spencer tested popcorn and other food to see if the magnetron could heat it up. To protect his idea to use a magnetron to cook food, he secured a series of patents. One of his early prototypes stood five feet tall. Over time, the technology advanced and microwaves became the small wonders that occupy space in many homes.
Ensuring the Safety of Microwave Ovens
Multiple safety mechanisms ensure the microwaves can’t escape their metal cage and harm you. For one thing, most microwaves automatically shut off the magnetron if the door opens thanks to an interlock. Many microwaves have two interlocks to reduce the risk that the magnetron could continue running with the door open. Even so, it’s best to avoid using your microwave if it appears as though the door isn’t shutting properly.
Modern microwaves come with a wide range of convenience and performance features. If you’re still using an older model, you may want to explore the latest innovations that make this small wonder even more useful. View our Microwave Ovens 101 guide to learn about the different types of microwaves on the market.