How a Microwave Oven Heats Food

A microwave oven heats food by filling a metal box with invisible radio-type waves. Water and other polar molecules in the food absorb the energy of those waves and warm up. The part that makes the waves is a vacuum tube called a magnetron, and the frequency most home ovens use is 2.45 gigahertz (GHz), which means the wave's electric field reverses direction about 2.45 billion times every second.

The magnetron makes the waves

The magnetron turns electrical power into microwaves. According to the Wikipedia article on microwave ovens, the cavity magnetron was developed in 1940 by British researchers Randall and Boot at the University of Birmingham, originally for radar. RF Essentials, a technical reference site, says the magnetron in a household oven produces roughly 1 kilowatt of microwave energy and is fed by a high-voltage transformer working at about 4 to 5 kilovolts.

From the magnetron, the energy travels along a metal channel called a waveguide into the cooking cavity. The cavity is a closed metal box. Metal reflects microwaves, so the waves bounce around inside instead of escaping, and the food is the main thing that absorbs them.

Why water gets hot

A water molecule is polar: one end carries a slight positive charge and the other a slight negative charge. When an electric field is applied, the molecule turns to line up with it. In a microwave oven the field keeps flipping direction, so the molecules keep twisting back and forth. That motion and the friction with neighbouring molecules release heat, which spreads through the food.

Other polar molecules respond too, which is why fats and sugars also warm up, though less strongly than water. Dry, non-polar materials such as most plastics and dry ceramics absorb little, so they stay cooler unless the hot food heats them by contact.

Why 2.45 GHz, and a common myth

Many people repeat that 2.45 GHz is the frequency at which water resonates. Both sources used here say otherwise. RF Essentials states that water's resonance is nearer 20 GHz, and the Wikipedia article says heating does not depend on a special resonance, because polar molecules respond across a broad range of frequencies.

The practical reasons are these, according to those sources:

• 2.45 GHz sits in an ISM band, a range set aside worldwide for industrial, scientific and medical use that does not need a licence.
• The frequency penetrates food by a useful distance: higher frequencies would heat mostly the surface, lower ones would pass too deeply.
• Magnetrons for this frequency are affordable to make.
• The US Federal Communications Commission approved the 2450 MHz frequency on 30 December 1946, according to the Wikipedia article.

Larger industrial ovens often use 915 MHz instead. That lower frequency penetrates deeper, but the Wikipedia article notes it is an ISM band only in some regions of the world.

How deep do the waves go?

Sources give different numbers here. RF Essentials gives a penetration depth of about 2 to 3 centimetres for typical food, while the Wikipedia article says that at 2.45 GHz the waves go only about 1 centimetre into most foods. The difference likely depends on what the food is made of, since water content, salt and temperature all change absorption. Either way, the depth is small compared with a large dish.

That is why the middle of a thick or frozen item is mostly warmed by conduction, meaning heat flowing inward from the outer layer, rather than by the waves reaching it directly. It also explains why the defrost setting uses low or pulsed power: pauses give heat time to spread into still-frozen areas.

Why spots stay cold

Inside the box the waves form patterns, with places where the energy is strong and places where it is weak. Food in a weak spot heats slowly. Two common fixes are a turntable, which carries the food through the pattern, and a stirrer, a small rotating metal fan that scatters the waves. The Wikipedia article adds that a turntable can still leave dead spots, such as the centre, and that it reduces the usable space to the area above the plate.

How the door keeps waves in

The metal case and the mesh in the door window work like a cage that blocks microwaves. According to the Wikipedia article, the holes in the mesh are much smaller than the wavelength of about 12.2 centimetres, so the waves cannot pass through them. Special choke structures around the door seal the gap at microwave frequencies, and interlocks cut power to the magnetron when the door opens. The same article reports that the US limit for leakage is 5 milliwatts per square centimetre at about 5 centimetres from the oven's surface.

A short history

The Wikipedia article reports that in 1945 Percy Spencer, an engineer at Raytheon, noticed that a radar set's microwaves melted a candy bar in his pocket. He tried popcorn and an egg next, and Raytheon filed a patent application on 8 October 1945. The company showed two Radarange units to the press in New York in October 1946, and a commercial Radarange followed in 1947. That first unit stood about 1.8 metres tall, weighed around 340 kilograms and was water-cooled.

Home use came later. The same source mentions a Tappan home model in 1956 priced around $1,250, a turntable added by Sharp between 1964 and 1966, and a $495 countertop model from Raytheon's Amana brand in 1967. RF Essentials describes the microwave oven as the highest-volume microwave application, with over one billion units in service worldwide.

Key takeaways

• A magnetron makes microwaves at about 2.45 GHz and a waveguide sends them into a metal box.
• Polar molecules, mainly water, absorb the energy and turn it into heat.
• The frequency was chosen for licence-free use, useful penetration and cost, not because water resonates there.
• Heating is uneven, so turntables or stirrers move food or waves around.
• The waves penetrate only a few centimetres, so the centre of thick food warms by conduction.

This article is general information. Sources disagree on some figures such as penetration depth, so treat numbers as typical rather than exact for every oven and food.

Sources

• Microwave oven, Wikipedia
• Microwave Oven, RF Essentials RF Glossary