Microwaves heat food quickly and efficiently using microwave technology, which was discovered by accident during World War II. Microwaves heat food by resonating with water molecules, causing them to spin and generate heat. This heats only the food, not the bowl or walls inside the microwave.
“30 seconds in the microwave”. This is a common advertising phrase for many instant meals these days. How did the microwave come to be so fast and convenient? Microwaves were invented by accident during World War II by Percy Spencer, an American scientist who was working on radar. While studying radio waves to improve his radar, he realized that the candy in his pocket had melted because of the waves, so he developed the microwave. The ability to heat food quickly and efficiently has made microwaves one of the most common and essential household appliances.
But how does a microwave work to heat food without heating the walls or air inside? To understand this, we first need to know about the “microwaves” that are generated inside the microwave. Microwaves are electromagnetic waves, a type of light. Light is an electromagnetic field traveling in space in the form of waves. Because light is in the form of waves, it has many similarities to waves as we know them.
Using waves as an example, let’s look at two important characteristics of waves: frequency and wavelength. The frequency is how often the waves come, and the wavelength is the distance between them. To make this easier to understand, imagine a ship in the ocean being bobbed up and down by waves. If the waves are closely spaced, the boat will bob up and down frequently. In this case, the closely spaced waves mean that the wavelength is short, and the frequent rocking of the boat means that the ‘frequency’ is high. Longer wavelengths mean longer intervals between waves, so in general, the larger the wavelength, the smaller the frequency.
Light, a type of wave, is categorized by its wavelength or frequency, and light with wavelengths between 1 millimeter and 1 meter is called microwave. If you consider that the visible light we perceive with our eyes has a wavelength of 0.00039 mm (violet) to 0.00079 mm (red), you can see that it has a very long wavelength compared to visible light. As we saw in the wave example above, the longer wavelength means that the frequency of microwaves is smaller than that of visible light.
It is these microwaves that actually heat the food inside the microwave. The microwaves heat the food by heating the water molecules, which are present in large amounts in most foods, which is why the food is heated without heating the bowl or the inside of the microwave. The principle of how microwaves heat water molecules without heating anything else is called resonance. Recently, the upper floors of the Technomart in Seoul vibrated and people were evacuated because the vibrations caused by dozens of people exercising in the gym were amplified by the resonance phenomenon.
‘Resonance’ is a phenomenon that occurs when a periodic force is applied to an object, and the vibration of the object becomes very large because the period of the periodic force coincides with the vibration of the object. The frequency at which the object vibrates is called the ‘natural frequency’. To understand this phenomenon, let’s take the example of pushing a swing. To get a bigger swing, it’s not only important to push hard, but also the timing of the push. No matter how much force you apply, if you push in the opposite direction of the swing’s motion, the swing will stop. If you use a small amount of force, but apply it periodically when the swing is at its back end, you’ll be able to make the swing get bigger and bigger.
The natural frequency is the number of times the swing oscillates in a given period of time. The natural frequency is a unique value that is determined by the properties of a material, and in the case of a swing, it is determined by the length of the rope. Longer swings take longer to vibrate once than shorter swings, so they have a lower natural frequency. To get a big swing, you need to time the push with the swing, so you need to push a short swing more often than a long swing, which has a higher natural frequency. When the frequency of the pushing force matches the natural frequency of the object, the object will vibrate strongly due to resonance.
The principle of heating water molecules is similar. First, let’s look at the shape of a water molecule. A water molecule is composed of one oxygen atom and two hydrogen atoms, and the oxygen atom is charged with a (-) pole compared to the hydrogen atom, so the oxygen atom is the (-) pole and the hydrogen atom is the (+) pole of the water molecule as a whole. Due to the presence of these (+) and (-) poles, the water molecule is forced by light, which causes the water molecule to rotate, i.e., the water molecule becomes a “swing” and the light applied to the water molecule becomes the “pushing force” on the swing.
The water molecules, energized by the light, rotate like a swing. Since the natural frequency at which the water molecules rotate is the same as the frequency of the microwave, the water molecules resonate with the microwave, causing them to vibrate very loudly. In the case of visible light, the frequency is too large to cause resonance with the water molecules, meaning that the swing is slowed down by applying a force before it reaches the back of the swing. The water molecules that are made to spin so much by the microwaves collide with other molecules around them and generate thermal energy, which is what heats the food.
This principle of microwaves is especially useful for busy people. For example, office workers prefer microwaves because they can quickly heat up food during their short lunch breaks. Students also often use microwaves to make quick snacks. Thanks to these conveniences, microwaves are widely used not only at home, but also in offices, schools, and other places. If you understand how microwaves work, you’ll understand why they only heat the food, not the bowl and walls.
The use of these invisible electromagnetic waves may not be familiar to the average person, but they’re already pervasive in our lives. Just as the principles of light and resonance are hidden in microwaves, it’s fun to discover the scientific principles behind everyday phenomena. Science is all around us, and understanding its principles can enrich our daily lives.
