Like a Regular Wave, Only Smaller

In my last article about lasers, a reader pointed out that in the history of the laser started off as something called a MASER. These devices were identical in functionality to a laser, except the beam was composed of microwave radiation, rather than visible light energy. In researching for my article, I had actually come across this fact, and I thought I mentioned it, but in re-reading the article, it looks like it slipped through the edits.

Obviously it’s easier to conceptualize lasers you can see. A MASER would be invisible since we can’t see microwave radiation. While the MASER didn’t really proliferate in the same way that lasers have, microwave radiation is one of the many things that makes our modern world work the way it does. To start off, let’s go to the obvious place, your microwave oven. Every single microwave oven in the world uses microwaves to heat food, but how does it work? First, we have to define what microwaves are.

Microwaves are electromagnetic radiation (yup, that same ol’ spectrum). They are defined as having wavelengths between one meter and one millimeter and frequencies between 300MHz and 300GHz respectively. Microwave radiation at about 2.45GHz has a special property though. When it passes through dielectric molecules, like water, it causes them to rotate. This rotation is the absorption of the microwave energy by the molecules. If you remember back to an article I did on cooling and temperature, the heat we see in the molecules is the result of them now moving. So the microwave oven isn’t heating your food, it’s heating any water or moisture present in the food. You can actually see the wavelength of your microwave by removing any turntable and placing a plate of shredded cheese inside. Turn the microwave on for approximately 30 seconds and take the cheese plate out. You’ll notice that there are rows of melted and unmelted cheese. The space between the melted rows is the approximate wavelength of the microwave.

Heating food isn’t the only use of microwaves though. Microwaves are the primary radio waves used for all modern communication. L band microwaves, from 1-2GHz are used for GPS, GSM mobile phones and amateur radio. S band microwave, from 2-4GHz are used for weather radar, microwave ovens, other mobile phones, WiFi, Bluetooth, and more GPS. The automotive radar I wrote about a few weeks ago is also a microwave. Some automakers will even call it “millimeter wave radar”. These microwaves are in the Q band ranging from 33-50GHz. Microwaves, unlike lower frequency radio waves, operate by line of sight and as such they are excellent for point-to-point communications.

I wonder if anyone caught something in that last paragraph. “Hadley, are you saying that the same band used for microwave ovens is used for cell phones?!?!” Yes, it’s true. Many mobile phones operate in the same frequency as microwave ovens. This is partially what caused a lot of concern when cell phones first started to proliferate. Is there a risk? Technically speaking, yes, but the risk is so incredibly minute, it would take decades of talking on the phone with it right to your ear before your chance of experiencing negative effects even rose a little bit. The reason for this is power. You may know how much power your microwave oven is rated for. Many are in the 1200 watt range. That’s 1.2 kilowatts of power. That’s a lot of power. That’s more power than a hair dryer might use, and the only way for most of us to get that power is to plug into the grid. A tiny cell phone battery is not capable of producing even a fraction of that power, and devices like WiFi transmitters are regulated to only transmit a certain amount of power.

Finally, why do we even call it radiation? Microwaves can’t cause chemical changes by ionization so they aren’t radioactive like plutonium is radioactive. This is one of the times we use “radiation” to mean energy radiating from a source. Technically you could call the light from a flashlight “light radiation”. The radiation itself is non-ionizing and therefore is not dangerous. The only danger from microwaves is if they are backed by sufficient power. Then your body could experience dielectric heating, just like in a microwave oven.

Hopefully this was a helpful discussion of what microwaves are and what they aren’t, and maybe I’ve removed some of the mystery of certain technologies that use them.