Microwave Ovens Are Not Just Kitchen Appliances

While for most of us microwave ovens are nothing but kitchen appliances used to “nuke” our breakfast, for Dr. Amarie’s students in Studio Physics 2 (Electricity, Magnetism and Optics) they are laboratory instruments. For them, the oven is a microwave resonant cavity capable of sustaining stationary electromagnetic waves (schematics, Figure A) in the microwave range characterized by nodes (cold spots) and anti-nodes (hot spots) similar to a vibrating guitar string when we play a note. But how to make the invisible waves visible, is the question? Well, we chose a very delicious approach: a platform covered first with a pattern of marshmallows (Figure B), then with Hershey’s chocolate bars “revealed” the hot spots as they melted (Figure B – inserts) and measurements were possible. To our surprise, given such a crude method, we determined a wavelength of 12.5 cm (5 inches), give or take a marshmallow, which comes within about 2% of the theoretical value of a 2.45 GHz microwave oven. As you can imagine the demo was a blast, we had to take advantage of the situation and since graham crackers started to show up out of the well-prepared students’ backpacks, the s’mores time was on! See? Physics is so sweet!

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Safety Comes First

Microwave Irradiation is concerning us all. Dr. Amarie’s students in Studio Physics 2 (Electricity, Magnetism and Optics) were excited to learn about the microwave radiation levels to which we are exposed to in diverse situations. But even if we would have means to perform experimental measurements, how can we tell when enough is enough? What are dangerous exposure levels? The instructor guided his students towards federal agencies like FDA and CDC which have set standards for microwave radiation safety since the 70’s. For example, we learned together that microwave ovens have a “safety interlock […that ] shall prevent microwave radiation emission in excess of 5 milliwatts per square centimeter at any point 5 centimeters or more from the external surface of the oven” [FDA – Code of Federal Regulations, Performance Standards for Microwave and Radio Frequency Emitting Products. (2015) 21CFR1030]. Armed with valuable knowledge, we started to explore our environment. As expected, we found the largest microwave intensity close to the oven. Our findings showed that in front of the oven’s door the radiation is maximum, and about 0.2 mW/cm^2, which is still 25× less than the federal requirement for safety. Next, we explored our classroom. Numbers seemed to vary quite randomly with values between 2.0 – 10 nW/cm^2, no matter the location: close to the fuse box, the PC monitor or under classroom projectors. Such measurement were very close to natural background measured in the park in front of the Math/Physics Building, roughly 1.0 nW/cm^2. Since all our cell phones emit in the microwave range, the next biggest attraction was about how much exposure we get when we make a call? To answer such question, over 50 students simultaneously called instructor’s office phone, than immediately placed their smartphones, on top of each other, with in 2-3 inches of the detector. Students’ learned that every time we make a phone call, we are exposed to an average microwave intensity of about 5 uW/cm^2 or 1,000 times less than the federal requirement. Let’s hope these experiments put some minds at rest.

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