How Do Microwaves work?

Originally published on Nov. 12th, 2015


Image courtesy of Monkey Business Images


How exactly does the magic box of deliciousness work? When you put cold or even frozen material into it and press some buttons, out comes something hot and ready to consume. Although it feels like something out of Harry Potter (or maybe the Jetson’s, depending on your pop-culture tastes), it’s just a matter of energy (Ha–science puns). First we have to address a teensy bit of physics: A microwave (the frequency, not the appliance), is on the electromagnetic spectrum, which means that along with all the light we can see, ultraviolet light, and x-rays, it is a kind of radiation. (‘Electromagnetic’ refers to any physical interaction between electrically charged particles). All that ‘radiation’ means, according to the World Nuclear Association, is ‘energy traveling through space’–in this case, in the form of a wave. Let’s break it down even more–what is energy? Specifically, energy is the ability to move or elicit change in matter. So, in short, a microwave is a kind of radiation that has the power to elicit a change in matter by using its charged particles to interact with other charged particles. Still sounds pretty magical.

The electromagnetic spectrum. Copyright:  Designua

So how did we entrap this amazing power and bend it to our own will? Namely, to make pizza pockets edible and to reheat that cup of tea you forgot on the counter? Microwaves occur naturally as part of solar radiation (our sun and all stars emit all the wavelengths of the electromagnetic spectrum at once). Most microwaves originating from the Sun get damped by our atmosphere, so little of this radiation trickles down to the surface of the Earth. The most important event that brought microwaves to the forefront of modern innovation and changed the way we thought about the universe was in 1964, when scientists Arno Penzias and Robert Woodrow Wilson at Bell Laboratories accidentally discovered cosmic background radiation (CBR). This is the pervasive low-level ‘hum’ that exists everywhere in space that is thought to be the leftover microwave radiation from the Big Bang. In other words, CBR is the echo of the beginning of the universe. As cool as that is, microwaves also went on to less philosophically charged fame.

A popular anecdote has it that a scientist at Raytheon Labs, Percy Spencer, was building some radar equipment that emitted microwave radiation and while testing it, found that the chocolate bar in his pocket was starting to melt. Upon further investigation, he found that corn kernels popped and eggs exploded when directly targeted with microwaves. Being business-savvy, he patented and commercialized the idea, bringing into being the first-ever microwave in 1947. (As a side-note, microwave frequencies can also be used for communication, navigation, astronomy, and spectroscopy).

In a modern microwaves, a device called a ‘magnetron’ converts electricity into microwaves (I know, it sounds like bad science fiction). The fluctuation of  microwave between its peaks and valleys also alternates its electrical charge. Many molecules in food, especially water, have dipoles, meaning that they have a partial positive charge at one end and a partial negative charge at the other. As the microwaves pass through the food, any molecules with dipoles align themselves toward the wave–the positive end aligns itself toward a passing negative charge and then as the wave changes, the negative end aligns toward the subsequent passing positive charge. These rotating molecules bump into other rotating molecules, causing a big mess of bouncing molecules–otherwise known as heat. Foods with less charged dipoles, such as most lipids and some sugars, are harder to heat up in a microwave, but when included with other foods, can absorb the heat from the rest of the meal that is successfully being heated via microwave.

The microwave radiation in a commercial appliance exists at a frequency that is non-ionizing. This means that is doesn’t change the fundamental nature or structure of any molecules or tissues (unlike the ionizing radiation like x-rays and UV rays) and so is not carcinogenic, though direct microwave exposure could cause damage to tissues by heating. Some plastics may not be safe to microwave as the radiation releases harmful chemicals and allow it to pass into the food it may be containing, so always check to make sure your containers are microwave safe. Ever wondered why the front door of a microwave sometimes has squiggly lines on it? That’s there to keep you safe! Those lines are a mesh whose perforations are smaller than the microwaves’ wavelength (12.2 cm for the usual 2.45 GHz), and so block the microwave radiation from passing through the door, while letting visible light (with its much shorter wavelength) through.

So there it is–anything you may possibly desire to know about your household microwave. Let me know your comments and questions in the comments below, or ask me on Twitter @thatlunchbxgirl