One of the fundamental scientific quantities in the universe is the speed of light. It is a huge number—about 300,000,000 meters per second or 186,000 miles per second—and is basic to electricity, magnetism, particle physics, cosmology, and the theory of relativity.
And, what’s really cool, most homes nowadays have the tools to measure it!
We tried what Jerome Pohlen suggested in Albert Einstein and Relativity for Kids and measured the speed of light in our microwave using a bar of chocolate. As a physicist, I find this one of the most fascinating home experiments I have ever heard of, and it’s not difficult at all.
- A bar of chocolate—we used a long, plain bar of dark chocolate that weighed 400 g (about a pound), although it really only needs to be about 5 or 6 inches long.
- A microwave.
- A ruler, either in inches or centimeters.
- Two paper plates—don’t try to use real plates; they absorb the radiation and give incorrect results by melting the chocolate with their heat. That is why we had strange results the first two times we did the experiment.
- A calculator or computer
Now, microwaves are a form of light, and light is a wave as shown below.
Your goal is to measure the wavelength of the waves in your microwave by seeing where the chocolate melts.
- In order to do this, you need to remove the turn table in your microwave. (It’s there to even out the hot spots that you do not want in your food but do want to measure in this experiment.)
- You should be able to put a paper plate upside down over the spinner that turns the turntable. Then you put second paper plate right side up on the first one, and place the unwrapped chocolate bar on it as shown below. (Note, we used real plates in this photo, and that was a mistake. Be sure to use paper plates.)
- Now you turn on the microwave for about 25 seconds, and when you take the chocolate out you should see two melted spots. If you can’t see them right away, flip the chocolate bar over and look on the other side.
- Measure the distance between the spots. This is the wavelength of the waves in your microwave oven. (Note that you will be guessing a bit, since the melted spots cover a bit of space. Just do the best you can, remembering that all experiments have such uncertainties.)
- Next you need to find out how many up-and-down cycles your microwave’s waves go through every second. This number should be written on your microwave, either on the door’s edge or on the back, and will most likely be 2.45 GHz or 2,450,000,000 up-and-down cycles per second. This is the frequency of your microwave.
- If you multiply the wavelength and the frequency together you will get the distance light travels in a second. (Use your computer if your calculator cannot handle such large numbers; older students can use scientific notation and a scientific calculator.)
- If you measured the wavelength in centimeters, you should get about 30,000,000,000 centimeters/second or, dividing by 100 because there are 100 cm in a meter, you should get about 300,000,000 m/sec.
- If you measured the wavelength in inches, you should get about 11,760,000,000 inches/second. To convert this to miles/second, you need to divide by 12 (to get feet per second, because there are 12 inches in a foot) and then divide by 5280 (to get miles, because there are 5280 feet in a mile). You should get about 185,000 miles/second.
Congratulations! You have measured the speed of light in your very own kitchen! Enjoy eating your chocolate bar to celebrate.
What if your number is close but not quite right? This is most likely due to the size of the melted spots on the chocolate bar. You could not really measure the distance between the spots accurately, and that is reflected in the fact that your numbers are slightly out. Such errors in experimental results are common in science; you’ve done the experiment correctly and have determined the speed of light as well as possible with a chocolate bar. (We got 174,000 miles/second instead of 185,000 miles/second because our measurement of the chocolate spots was out by .3 inches.)
What if your number is way off? The first two times we did the experiment our numbers were way, way off. Once they were double what we expected, and once they were half. We had used pottery bowls the one time and china plates the other time, and they had gotten warm in the microwave and caused the chocolate to melt at the wrong places. When we used paper plates, the experiment worked for us. So, if your numbers are way off this may be your problem too.
Other possible reasons for getting a number that is way off: Perhaps you forgot to check your microwave oven to be sure that its frequency is indeed 2.45 GHz. Perhaps you used the wrong numbers when you did your calculations or you multiplied instead of dividing. If all of these factors are correct, please leave a question in the comments.
Note: We learned about this experiment from Jerome Pohlen’s excellent book Albert Einstein and Relativity for Kids.