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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Download Student Worksheet & Exercises


First, you’ll need to find the ‘hot spots’ in your microwave.  Remove the turntable from your microwave and place a naked bar of chocolate on a plate inside the microwave.  Make sure the chocolate bar is the BIG size – you’ll need at least 7 inches of chocolate for this to work.  Turn the microwave on and wait a few minutes until you see small parts of the chocolate bar start to bubble up, and then quickly open the door (it will start to smoke if you leave it in too long).  Look carefully at the chocolate bar without touching the surface… you are looking for TWO hotspots, not just one – they will look like small volcano eruptions on the surface of the bar.  If you don’t have two, grab a fresh plate (you can reuse the chocolate bar) and try again, changing the location of the place inside the microwave.  You’re looking for the place where the microwave light hits the chocolate bar in two spots so you can measure the distance between the spots. Those places are the places where the microwave light wave hits the chocolate.


Open up the door or look on the back of your microwave for the technical specifications.  You’re looking for a frequency in the 2,000-3,000 MHz range, usually about 2450 MHz.  Write this number down on a sheet of paper – this tells you the microwave radiation frequency that the oven produces, and will be used for calculating the speed of light. (Be sure to run your experiment a few times before taking actual data, to be sure you’ve got everything running smoothly.  Have someone snap a photo of you getting ready to test, just for fun!)


When you’re ready, pop in the first food type on a plate (without the turntable!) into the best spot in the microwave, and turn it on.  Remove when both hotspots form, and being careful not to touch the surface of the food, measure the center-to-center distance using your ruler in centimeters.


TIP: If you’re using mini-marshmallows or chocolate chips (or other smaller foods), you’ll need to spread them out in an even layer on your plate so you don’t miss a spot that could be your hotspot!


How to Calculate the Speed of Light from your Data

Note that when you measure the distance between the hotspots, you are only measuring the peak-to-peak distance of the wave… which means you’re only measuring half of the wave.  We’ll multiply this number by two to get the actual length of the wave (wavelength).  If you’re using centimeters, you’ll also need to convert those to meters by dividing by 100.


So, if you measure 6.2 cm between your hotspots, and you want to calculate the speed of light and compare to the published value which is in meters per second, here’s what you do:


2,450 MHz is really 2,450,000,000 Hz or 2,450,000,000 cycles per 1 second


Find the length of the wave (in cm): 2 * 6.2 cm = (12.4 cm) /(100 cm/m) = 0.124 meters


Multiply the wavelength by the microwave oven frequency:


0.124 m * 2,450,000,000 Hz = 303,800,000 m/s


Published value for light speed is 299,792,458 m/s = 186,282 miles/second = 670,616,629 mph


Click here to learn how to turn this project into a Science Fair Project you can enter!


Exercises


  1. What would happen if you used cheese instead of chocolate?
  2. Does it matter where in the microwave the chocolate is located? Does placement of the chocolate affect the wavelength?
  3. Can you explain what the burn marks on the chocolate bar are from?

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Comments

21 Responses to “Measuring the Speed of Light with a Chocolate Bar”

  1. That’s still fantastically close given that we’re using a chocolate bar and a microwave of various accuracy! You can try doing this experiment 3 times and average the final results to give an overall answer, which may be closer.

  2. Stacey Morin says:

    Is it okay if the published value is greater than the measured value? In our case it was. For the % difference we did published value (299,792,458) – measured value. Our % difference was 13.4%. What do you think we could have done differently?

  3. Try moving the chocolate bar around – the second hotspot might be just beyond the edge of the bar.

  4. Adilah Muhammad says:

    Our chocolate bar is only producing one hotspot. Are there supposed to be two hotspots? Our chocolate bar is 5.5 inches. Does this effect the outcome of the hotspots? Will we be able to continue with only one hotspot?

  5. Brenda Lykins says:

    Very fun experiment and interesting. Great for visual/hands-on learners. Have some strawberries on hand- after we completed our measurements, we dipped strawberries in the chocolate that was not burned by the hotspots-Yum!

  6. Great question! The microwave makes light waves with a frequency of 2,450,000,000 Hertyz (cycles per second). Once cycle is when you go from peak to peak on a wave, and you can see this with the distance between the bits of chocolate that melt. Multiply the cycles per second by something measured in meters per cycle gives meters per second, which is the wave’s velocity, or the speed of light. Scientists make educated guesses about what they think will happen, and then do an experiment and observe the results so they can better understand the world!

  7. What do the hotspots on the chocolate represent relative to the waveform & frequency? What I am asking is, why does this experiment work (or come so close)? How does a scientist KNOW that this experiment will correlate to the speed of light? Thanks. LC

  8. Ed Endemano says:

    OK, thank you

  9. No, 60 Hz is at the wall outlet. You want the number for the microwave output. Did you check under it or inside the door panel?
    Do you live in the US? If so, use the numbers recommended in the video.

  10. Ed Endemano says:

    Aurora,
    Our microwave isn’t very old and it doesn’t seem, but it said 60 Hz we couldn’t find any thing else we don’t know what to do? Please help us!

  11. Unless it’s a really old or unusual microwave, most are 2450Mhz as shown in the video. Sometimes the sticker comes off. Try using this number and see how close you get to the answer for c.

  12. Heather Henry says:

    Our problem is that there is NO listing of MHz on our microwave. It only lists 1.5 K/W. Advice?We even called GE and they don’t have any information on what the MHz is. Help! Advice?

  13. I am curious – what happens if you elevate the plate, like putting it on two or three water glasses during the experiment? Do you get the same value?

  14. Nora Taylor says:

    We too, consistently get 9 cm between hot spots using our 2450MHz microwave.

  15. 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!

  16. Jean Custer says:

    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.

  17. 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.

  18. 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.

  19. Jean Custer says:

    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. ??????

  20. Jean Custer says:

    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
    :

  21. sevy keble says:

    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 😀

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