Microwaves are a very useful cooking device. They're compact, tiny, but incredibly versatile machines – so how do they work?
There's an incredible amount of science and engineering behind microwaves, but it can all be broken down into easy to understand and digest bits.
Microwave ovens mainly function by generating microwave radiation - which is passed through the food, cooking it. Microwaves aren't specific to microwave ovens, rather they are a form of electromagnetic waves with a wavelength in the frequency range of between 300 MHz and 300 GHz (a wavelength of around 1 m and 1 mm). Microwave ovens generally use a frequency of 2450 MHz (a wavelength of 12.24 cm).
When compared to other types of radiation, microwaves fall between radio waves, which tend to be longer, and infrared waves, which are shorter.
What makes microwaves so specialized for cooking is just how they interact with water molecules.
The science behind microwaves
Inside microwave ovens, the microwaves are created using a device called a magnetron. At their core, magnetrons are high-voltage motors that output energy in the form of microwaves rather than mechanical work.
Inside the microwave oven, a transformer steps up electricity from the wall socket, 120V in the US, or 110V in Europe, to around 4000 volts. This increased voltage is needed to power the magnetron, causing a filament to heat up at the core of the device. As this filament heats up, electrons are released. The microwave oven puts these electrons to work.
A circular magnet is located near the heated filament. Normally, the electrons that are released by the filament would become attracted to the anode, but because of the positioning of the magnet, the electrons loop back to the filament itself. This looping of electrons is what actually creates microwaves.
All of that may have sounded a little bit complex, and it can be when explained without diagrams. For a deeper dive into the understanding of how magnetrons and, ultimately, microwaves work, take a look at the video below.
Are you safe to stand next to a microwave?
One popular myth about microwaves is that they can give you cancer. This may stem from the use of the word 'radiation' in describing how they work, as well as a fear that this microwave radiation can leak out of the microwave. Radiation, in this case, refers to energy that radiates from a source, and not to radioactivity.
However, microwaves do not contain enough energy to chemically change substances through ionization - they are an example of non-ionizing radiation. Other types of electromagnetic waves such as ultraviolet and x-rays possess more energy per photon and thus can cause cancer.
Microwaves can, of course, cause heating and burns, but microwave ovens are all designed with a metal mesh on the door of the appliance to keep the microwaves from leaking out. These metal meshes are large enough for you to see your food cooking, but the holes aren't large enough for the microwaves to slip through. Microwave ovens also have built-in safety devices that do not allow the oven to work while the door is open.
So, at the end of the day, you won't get cancer from standing too close to a microwave because the rays aren't ionizing, and you won't get cooked either.
So, we've gotten through the basic science, but we still haven't covered why exactly microwaves work so well to heat food quickly.
How microwaves heat up water molecules
The microwaves used in a microwave oven are sent out through a type of antenna that channels them into the cooking area of the appliance. The waves are contained inside of the appliance thanks to the solid metal walls (and mesh door). These microwaves are then absorbed by the water molecules inside of the food. The energy from the microwaves causes the water molecules to vibrate rapidly, which heats the food.
In essence, because water molecules easily absorb microwaves, which cause the molecules to start vibrating rapidly, this becomes a mechanism to turn microwave energy into thermal energy. Just like a cam and roller is a mechanical device for converting rotational energy into linear motion, so too are the interactions of microwaves and water molecules a way of converting microwave energy into heat energy.
But why are just the water molecules vibrating? Water molecules are polar, meaning they have a positively charged side and a negatively charged side. Microwaves have a positive crest and a negative crest, just like any wave does. As the microwaves move throughout the inside of the microwave oven, the water molecules will try to align their poles with that of the microwave. Since microwaves move about rapidly inside of the appliance, the water molecules rapidly try to keep aligning themselves with the motion of the waves.
Just how fast is this happening? Microwaves' positive and negative fields are moving at roughly 2.5 billion times per second. When you combine this effect with the water molecule's reaction, you can start to realize how microwaves cook food so fast and effectively.
Why do microwaves cook unevenly?
The last question we need to answer is why there are often dead spots on the inside of microwave ovens. For example, why is the center of your food often much colder than the edges?
This occurs because some of the microwaves end up canceling each other out. Oftentimes, these places of cancelation are localized, meaning some areas of the microwave oven don't get any "heat".
In essence, when the peak of one wave hits the valley of another wave, the result is a canceled-out wave. This is why most microwaves use a rotating dish in order to ensure even cooking.
So, that's how microwave ovens work. They are appliances that use magnets to produce electromagnetic radiation at a particular wavelength, which is directed at water molecules in food, causing them to vibrate and heat up rapidly. Next time you cook something in a microwave, you won't look at it the same way.