Measuring the speed of light with chocolate and a microwave oven

Here’s a great excuse to eat a lot of chocolate in the name of science.

Standard electromagnetic (light) wave.

First of all, you need to understand that microwaves are just electromagnetic waves with a certain frequency and wavelength just like visible light. Wavelength is the length between consecutive peaks of the wave. It’s a very intuitive name. You can see it labeled in the picture to the right as the Greek letter “lambda” (λ). Since waves propagate (move), we can also define a quantity called the frequency. Frequency is the number of peaks of a wave that pass a certain fixed point per second. Wavelength is a measure of distance, and frequency is a measure of one divided by time. So to find the speed (“c”) of the wave, you just need to multiply these two quantities together:

c := speed of light = (distance) x (1/time) = (frequency) x (wavelength)

…but why the chocolate and the microwave, you ask?

What you need to do is use a microwave oven and a piece of chocolate to measure the frequency and wavelength of microwaves. Then you can find the speed of light! Fortunately, microwaves usually have the operating frequency written on the back. Check the back of your microwave. Mine says the frequency is 2450 MHz ( = 2,450,000,000/1 second).

Now that we have the frequency, all we need is the wavelength; this is where the chocolate comes in handy… You might have a microwave with a spinning dish inside. You can probably guess what that’s for. It’s to help heat things up evenly (like stirring a pot of soup on the stove). Whereas on a stove the heat is concentrated on the bottom of a pot, the energy (and thus heat) that microwaves give to food is concentrated at the peaks of the microwaves (which are standing waves in a microwave oven). If we take out the rotating dish then we can find these peaks, measure the distance between them, and find the wavelength. So we just need to heat the chocolate up a bit, find some soft spots (where the peaks of the microwave standing wave are) and measure the distance between them with a ruler.

Here’s what you’ll need:

  • Large chocolate bar (bigger than 5 inches)
  • Ruler (to measure distances)
  • Microwave oven (with rotating dish removed)
  • Coffee (optional... it goes well with chocolate)

Place the chocolate bar (unwrapped) in the microwave oven and heat it up (without moving it) until you can see soft spots forming. If I were to hazard a guess for the timing, I’d say heat about thirty seconds… but that’s a guess. It really depends how powerful your microwave oven is.

mmm... gooey...

When you have at least two soft spots forming in the chocolate, take it out and try to measure the distance between them with a ruler. (I had to prod the chocolate lightly with a spoon to find the soft spots). My chocolate didn’t turn out very nicely, but I was able to make a very rough measurement of about 4.5 inches between the centers of the soft spots. Edit: when I made this measurement I forgot that I was measuring peaks of a standing wave which are half the wavelength of the microwave. So really, you should find the distance between the soft spots and multiply by 2 to get the wavelength. Thanks, Lord Axil. Somehow I must have missed a soft spot when measuring, which automatically corrected this factor of two.

Now we can use the wonders of Google to do the calculations for us. I can just type the following right in Google and it will calculate the speed in the proper units: “2450MHz*4.5inches”.

Google says:

(2450 megahertz) * 4.5 inches = 280 035 000 m / s

…but how close are we to the real answer, you ask?

We can check our answer with Google again. Just type “c” into google and it will give you the speed of light!

Google says:

the speed of light = 299 792 458 m / s

For a measurement made with a ruler and a chocolate bar, it’s not too far off. It works!

But don’t take my word for it, listen to this amateur scientist explain the experiment.