Measuring the Speed of Light with a Chocolate Bar
When you warm up leftovers, have you ever wondered why the microwave heats the food and not the plate? (Well, some plates, anyway.) It has to do with the way microwave ovens work.
Microwave ovens use dielectric heating (or high frequency heating) to heat your food. Basically, the microwave oven shoots light beams that are tuned to excite the water molecule. Foods that contain water will step up a notch in energy levels as heat. (The microwave radiation can also excite other polarized molecules in addition to the water molecule, which is why some plates also get hot.)
One of the biggest challenges with measuring the speed of light is that the photons move fast… too fast to watch with our eyeballs. So instead, we’re going to watch the effects of microwave light and base our measurements on the effects the light has on different kinds of food. Microwaves use light with a wavelength of 0.01 to 10 cm (that’s ‘microwave’ part of the electromagnetic spectrum). When designing your experiment, you’ll need to pay close attention to the finer details such as the frequency of your microwave oven (found inside the door), where you place your food inside the oven, and how long you leave it in for.
Materials:
- chocolate bar (extra-large bars work best)
- microwave
- plate
- ruler
- calculator
- pencil and paper
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The frequency stays the same, but you’ll probably notice something’s different with various power settings… which means you should test out BOTH and see if you notice any difference in the wavelength spread!
Thanks for the hints. We’ll try again as soon as we get some more test chocolate. Tim asks if it matters whether the microwave is set on high or low.
Good question. If the wavelength that hits your chocolate bar smacked it at the peak, then you’re right, it would be one wavelength. Most of the time, however, that chocolate bar is sitting in a spot that hits below the trough, usually in the middle (vertically), so you only get a half-wavelength measurement. The tricky part in visualizing this is that you can’t see the waves, and you have to guess where they hit in 3D space. You can elevate the chocolate bar using a glass under the plate while keeping it in the same spot and you’ll see that it still melts in the same spot.
You might be getting spots from two different beams. Move the chocolate around and see if you can find a spot where they are closer together.
We consistently get about 9 or 9.5cm between the hot spots, using our 2450MHz microwave. I wonder why our speed of light calculation is off by so much. ??????
I have a question about the wavelength calculation: If the peaks of the wave are at the hot spots on the chocolate, isn’t the distance from peak to peak (or hot spot to hot spot) a measure of the wave length? Could it be that the trough of the wave is causing a hot spot too?…..We’re having fun with this experiment, but that question is bugging us…Thanks
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We tried this expirement and “Wow!” We found out that the heat goes from the side of the microwave to the edge of the bar, bounces of, the goes to the other edge! Amazing! :l