A melted chocolate bar
In 1945, Percy Spencer, an engineer at the Raytheon company in the United States, was working near a magnetron, a device that produced microwaves for military radar. He noticed that a chocolate bar in his pocket had melted.
Curious, he tried other foods. Popcorn kernels popped. An egg exploded (onto a coworker’s face, according to the story!). Spencer realized microwaves could cook food quickly.
From radar to kitchen
| Year | Event |
|---|---|
| 1940 | British scientists develop a powerful cavity magnetron for radar during World War II |
| 1945 | Percy Spencer discovers microwave cooking; Raytheon files a patent |
| 1947 | First commercial microwave, the "Radarange": about 1.8 m tall, weighing about 340 kg, cost thousands of dollars |
| 1967 | Amana releases an affordable countertop microwave for homes |
| 1980s | Microwaves become common in Canadian homes |
| Today | Found in most homes, offices and staff rooms |
Spencer had no university degree. He left school young and taught himself electronics, eventually earning about 300 patents.
What are microwaves?
Microwaves are electromagnetic waves, like light and radio waves. They have wavelengths from about 1 mm to 30 cm. Microwave ovens use waves about 12 cm long (2.45 GHz frequency).
Microwaves are also used for:
- cell phones and Wi-Fi
- radar for aircraft, ships and weather
- satellite communication and GPS
How a microwave oven works
| Part | Job |
|---|---|
| Magnetron | Produces microwaves |
| Waveguide | Channels microwaves into the cooking chamber |
| Metal walls | Reflect microwaves so they bounce around inside |
| Turntable or stirrer | Moves food or waves around for even heating |
| Door with metal mesh | Lets you see inside while keeping microwaves in |
Heating with water molecules
Food contains water. A water molecule has a slightly positive end and a slightly negative end.
- Microwaves have an electric field that flips direction billions of times per second.
- Water molecules try to line up with the field, so they twist back and forth rapidly.
- As they twist and bump into neighbouring molecules, they create friction, producing heat.
- This heat cooks the food from the outside few centimetres inward.
Fats and sugars also absorb microwaves. Glass, paper and most plastics absorb very little, so they stay cooler (though hot food can heat them).
Why microwaves cook unevenly
- Microwaves form patterns of "hot spots" and "cold spots" inside the oven. The turntable helps even this out.
- Microwaves only penetrate a few centimetres, so thick foods heat from the outside in.
- Ice absorbs microwaves poorly, which is why defrosting can leave frozen and cooked spots side by side.
Tip: stir food partway through and let it stand for a minute so heat spreads evenly.
Safety
| Fact | Explanation |
|---|---|
| Microwaves don’t make food radioactive | They’re non-ionizing; they just make molecules move |
| The door’s metal mesh blocks microwaves | It acts as a Faraday cage: the holes are smaller than the 12 cm wavelength, so waves can’t pass through |
| The oven stops making microwaves when the door opens | Safety switches turn off the magnetron |
| Don’t put metal inside | Metal can cause sparks (arcing), especially foil and forks |
| Don’t heat sealed containers or whole eggs | Steam builds up and they can explode |
| Liquids can superheat | Water can heat above boiling without bubbling and erupt when disturbed |
Impact
- Fast reheating and cooking saves time and energy compared with heating a whole oven.
- Microwave popcorn became a huge snack-food industry.
- Hospitals, labs and factories use microwave heating for drying, sterilizing and other tasks.
Common mistakes
- Thinking microwaves cook from the inside out. They heat the outer few centimetres; heat spreads inward.
- Thinking microwaved food is radioactive.
- Thinking all containers are microwave-safe. Some plastics can melt or leach chemicals.
Try it yourself
- Remove the turntable (with an adult), put a plate of mini marshmallows or chocolate in, and heat for 20 seconds. Measure the distance between melted spots. Double it to estimate the wavelength!
- Heat a cup of water and a cup of vegetable oil for the same time. Which gets hotter?
- Make a poster of microwave safety rules.
Quick recap
- Percy Spencer discovered microwave cooking in 1945 when a chocolate bar melted.
- Microwaves are electromagnetic waves also used in radar, phones and Wi-Fi.
- A magnetron produces microwaves that make water molecules twist and heat food.
- Metal mesh in the door blocks microwaves.
- Microwaves heat quickly but can be uneven; follow safety rules.