Measure the Speed of Light with Chocolate!


With nothing more than a big bar of chocolate, a microwave oven, a ruler and a little bit of math, you can measure the speed of light for yourself, and then eat your results!

Microwaves are a type of electromagnetic radiation, just like light waves. Microwaves also travel at the speed of light. If you measure how fast they are travelling, you should get a result close to the speed of light.

First the math. To measure the speed of light, you need two measurements: wavelength and frequency because

speed of Light = wavelength X frequency

and we know that the speed of light is 299,792,458 meters per second.

FREQUENCY refers to the number of cycles per second, and  is posted on a sticker on the microwave, usually on the back or bottom of the microwave. Find the frequency of your microwave and write it down. Most standard microwaves have a frequency of 2.45 gigahertz.

WAVELENGTH is what we will measure with the microwave and the bar of chocolate.




As you can see on the diagram above, the wavelength is the distance from one crest to the next on a wave.  With our bar of chocolate and our microwave, we are going to measure the length of a microwave.


1. Remove the turntable from your microwave oven. This will allow the microwaves to hit the chocolate bar in the same place over and over again. The turntable turns the food so that the microwaves are more evenly distributed to heat your food evenly… usually that is what we want our microwave oven to do, but for this experiment, we’re going to allow the waves to “hit” the chocolate in the same place over and over so that there are marks that we can measure.

2. Unwrap your chocolate bar and put it in the microwave oven on high for about 20 seconds and then remove it (carefully!). You will notice several melted spots on your chocolate bar like in the picture below.

melted chocolate

melted chocolate

3. Measure the distance between two melted spots and multiply by 2, and write this number down. Why multiply by 2? Because there is a melted spot at both the crest and the trough of each wave, and remember, the wavelength is the distance from crest to crest or trough to trough. The distance between two melted points on your chocolate is only the distance from crest to trough.

4. Convert gigahertz to hertz! 2.45 gigahertz = 2,450,000,000 hertz

5. Calculate the speed of light!  This is the easy part! Plug your findings into the equation and solve for the speed of light.

the speed of light = the frequency of your  microwave oven X the distance between 2 melted spots X 2

Remember, if you measured the distance between the melted spots in centimeters, the speed of light will be in centimeters per second. To get an answer in meters per second, divide the speed of light by 100.

Was your answer close to the speed of light?

Now you’ve satisfied your curiosity, you can eat the chocolate. A well deserved reward for you hard work.