February 13, 2012

Measuring the speed of light in a microwave oven - is there a better way?

Did you see this video? I subscribe to the CoolStuff newsletter from Arbor Scientific.  They send out an email roughly once a month which usually has some sort of  interesting idea for a demonstration or lab activity that I can use in the classroom.

This month they included a video about how to measure the speed of light using a microwave oven. The idea is that if you put something soft in the microwave (I've seen this done with marshmallows or chocolate) and turn the microwave oven on until the first hint of melting occurs you can measure half of a wavelength of the electromagnetic wave (in the microwave portion of the EM spectrum) by measuring the distance between soft spots.  Multiply the wavelength by the frequency of the microwave wave (conveniently printed on the back of the microwave oven) and voila! you have the speed of light to within a few percent.

It's a great idea for an exploration into EM wave phenomena.  But, I was a little underwhelmed by the explanation given in the video and on the supplemental page.  I don't want to nitpick too much. Mainly, I was disappointed that the explanation given was very traditional in the way it was presented. Kind of like: "Here are the facts. Here are the equations. Here's the reason it happens. Here are our results."

There wasn't much discussion about the pattern left in the chocolate. I have questions about the video! Why was the pattern in the chocolate shaped the way it was? Was there something about the magnetron (or was it thewaveguide?) in the microwave oven that lead to this pattern?  Do microwave ovens still use magnetrons to generate microwave waves?  How does chocolate get heated in the microwave if it has almost no water content?  I thought heating of food was due to a resonance of water molecules in the foot.  (It's not.)

I took some notes on what bothered me about the explanation of the phenomena.  What did you notice about this video? Have you done this exploration in class?  How have you done it differently to make it more engaging?

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