​Appliance Science Experiments: Mapping your microwave hot spots

Do you want to know how good your microwave is? You can see how many hot spots it has with a few bits of buttered bread, and you can measure the speed of light at the same time. Appliance Science looks at mapping your microwave.

Colin West McDonald/CNET

Microwaves are tricky things: when you reheat your dinner in one, sometimes they make your food too hot, or bits of it stay cold. That's not really the fault of your microwave oven: it is more to do with the way that microwaves (and all electromagnetic radiation) behave. As these waves bounce around inside your oven, they cancel each other out (meaning cold food) or double up, which means very hot food. I discussed how microwaves work in a recent column, but I didn't get into the specifics of hot spots. So, I decided to try and map the hot spots in my microwave, and to see how well the various measures that the microwave employs to get around this work. While I was at it, I wanted to try to calculate the speed of light.

There are various materials that you can use to map the hot spots, using old-school methods like thermal fax paper, marshmallows and chocolate. I decided to use a slightly more mundane material: bread and a margarine spread that makes a bold (and, to me, rather overambitious) claim about tasting like butter. My microwave is a midrange Panasonic model that includes a large rotating dish, and Panasonic Inverter Technology, which it claims "delivers a seamless stream of cooking power."

4 pieces of bread, ready for experimentation
Four pieces of buttered bread, ready for experimentation Richard Baguley/CNET

My experimental material is four pieces of bread (whole-meal, of course) evenly coated in the aforementioned spread and arranged on a plate. I decided to use this because the spread is easy to melt, and it is easy to tell the melted from the nonmelted. After a few test runs, I figured out that a cooking time of about 20 seconds worked perfectly, as this meant that some (but not all) of the spread was melted.

Microwaved with the rotating dish on: nice even melting Richard Baguley/CNet

First, I tried this arrangement with the microwave running normally, including the rotating dish. As you can see, this produced a pretty even pattern of melted spread: the only spots that aren't melted are right at the edge of the slices.

Microwaved with no rotating dish: spotty melting. Richard Baguley/CNET

Next, I tried it with the rotating dish removed, so the plate did not rotate. That made a very big difference. See all of those spots of melted spread? Each one of those is a hot spot. Imbetween these, the spread is not melted.

The moral of this experiment? The dish in your microwave rotates for a good reason. It rotates so the food in there gets more evenly cooked, without hot spots that would mean, at best, cold food or at worst, food poisoning.

If you want to try this yourself, my bread and butter is a great indicator, but you could also try pappadams, a crunchy Indian treat that expands and gets crunchy when cooked. The wonderfully named Evil Mad Scientist Labs did this, with rather fascinating results. Plus, pappadams are very tasty, especially with chutney.

The hot spot pattern in a very cheap microwave. Evil Mad Scientist Labs

The speed of light

OK, so I now have several maps of the hot spots in my microwave. How can I use these to measure the speed of light? Simple: by applying physics. Specifically, by applying the equation speed = frequency x wavelength. First outlined by Maxwell in 1864, this equation connects the speed of an electromagnetic wave with the frequency and the wavelength. In a microwave oven, we know the frequency, because it is on a label on the back of the device: 2.45MHz, or 2,450,000,000Hz. One Hertz is one cycle per second.

I measured the space between the hot spots of melted spread on the bread. Richard Baguley/CNET

We can work out the wavelength of the microwaves from the pattern on our bread. Because the hot spots are caused by the peaks of two waves matching up, the distance between the hot spots is half of the wavelength (one wavelength is two peaks). So, I ran this experiment a few times and measured the distance between the center of each hot spot and its closest neighbor. This gave me an an average of about 2.3 inches. If we double that, we get a wavelength of about 4.6 inches.

So, let's plug these numbers in to our equation.

2,450,000,000Hz x 4.6 inches = 11,270,000,000 inches per second.

If we translate that into more usable measurements, we get 939,166,667 feet per second, or 177,782 miles per second. People with more accurate instruments than my bits of bread have measured the speed of light at 186,000 miles per second, so I wasn't too far off: just over 4 percent. That's not bad for a few pieces of bread and a back-of-the-envelope calculation....

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