So, how do microwaves work? In a word, resonance – Resonance is a phenomenon that occurs when frequency of force acting on an object matches its vibration.
This sounds complicated right? It’s not
It’s the same principle as pushing someone on a swing set, if you push them too fast or too slow, then it doesn’t work, but if you push them at just the right time, you end up adding a bit to their speed each time. This is exactly what is happening in the microwave.
The electromagnetic waves released by the microwave don’t transfer much energy to the food, just enough to vibrate the molecules slightly, but the frequency (or wavelength) happens to be just right to match the way that the water molecules in your food vibrate, meaning that each successive wave pushes the molecules slightly faster, heating them up.
The net result is that the microwave heats up the water in your food, but not the rest of it. The heated water cooks the food. This is why food cooks in the microwave without browning, and the microwave does not work on food with low water content, you are effectively steaming the food from the inside out.
So what does this mean for you?
Well you don’t need to worry about all of the scare stories about microwaves “nuking your food with radiation” the microwaves used are a form of radiation, but only in the sense that light, radiated heat, and radio waves are too.
Of course direct exposure to the microwaves could be harmful to you, but only in the sense that you contain water yourself and should probably endeavour not to allow that water to boil. The microwaves are kept safely inside the box by a metal cage that is sufficiently fine that the waves from the microwave cannot pass through; this is the fine grid that you see when you look through the microwave window, as long as this is intact the microwaves cannot escape.
Once the microwave is switched off the only thing that persists in the food is the heat, no “radioactive residue” is left behind. In fact the big difference between microwave cooking and normal cooking is that because the food is heated evenly you don’t get the caramelising “browning” effect that you would see from a grill. The chemicals that cause this effect are potentially worrisome themselves, meaning that food properly cooked through in a microwave may actually be safer.
Of course if you just use the microwave to cook sugary, salty, ready meals, then this is not terribly good for you, but the microwave itself is blameless for this.
One thing that is important to be aware of is that because the energy in the microwave is trapped and an empty microwave doesn’t contain any water to heat you must make sure that you never switch the microwave on with nothing inside or you can damage the machine.
You should also be careful when using a microwave to heat water, especially in a smooth container such as a glass. When water is heated it needs a surface on which it can form into a gas, this is called nucleation. It is possible, but rare, for a microwave to heat water past boiling point without this having a chance to happen, superheating the water, when nucleation occurs, usually when the container is moved the bubbles formed can themselves act as nucleation points causing a chain reaction resulting in an explosion of steam. This can be avoided by leaving an object such as a plastic spoon in the container as you heat it, allowing bubbles to form safely.
Lastly you should never place metal objects in a microwave as they reflect the microwaves unpredictably. Metal foil can be particularly dangerous as it will tend to focus the microwaves into hotspots.