How to Measure the Speed of Light... Using Chocolate!




In this Instructable, the first in a series using the book How to Fossilize Your Hamster And Other Amazing Experiments for the Armchair Scientist as inspiration, we use a bar of chocolate to measure the speed of light.

What you'll need:
A bar of chocolate, actually, get three, that way you know you'll actually get to do the experiment! (The longer the bar of chocolate, the better)
A microwave
A metric ruler
Safety Glasses (not that this is dangerous, it just adds awesome factor to any experiment)

-Bradley Powers

Step 1: Eat Some Chocolate!

You know you want to. You don't have to smear it all over your face though. In fact, I don't recommend it.

Step 2: To the Microwave!!!

Remove the rotating tray thingy from your microwave, we don't want the chocolate to cook evenly.

Step 3: Zap the Chocolate

Now, place the bar of chocolate in the microwave. Turn on the microwave, and wait for pools of chocolate to form, then turn off the microwave. It should take about 40 seconds. I'll wait. Don't overcook the chocolate, it doesn't smell so good.

Step 4: Measure

Now, take out the chocolate, and measure from "hot spot" to "hot spot". A "hot spot" is where the chocolate is starting to melt, or is more melted than the rest of the chocolate. Write the measurement down. Seriously. Do it.

Step 5: Now for the Mathy Stuff.

Ok, now that we know the distance between hot spots, we'll use some math, and some science, and some more math to figure out the speed of light. First, the distance that we measured represents the half-wavelength of the waves being emitted by the microwave (according to the book). To find the wavelength of the microwaves, we multiply by two. In my example, that gives us a wavelength of

7.628 cm * 2 = 15.256 cm

Now, since the speed of light is equal to the wavelength times the frequency, we can figure out the speed of light. But we don't know the frequency of the microwaves. Apparently, most microwaves operate at 2.45 gigahertz, or 2,450,000,000 Hz. So, we take the the product of the wavelength and the frequency:

15.256 cm * 2,450,000,000 Hz = 37,377,200,000 cm/s which, given that we are doing this in a kitchen (and a small error our measurements are multiplied by 4,900,000,000), is shockingly close to the actual speed of light, which is 29,979,245,800 cm/s, or, as it is typically defined, 299,792,458 meters per second.

Step 6: Iterate

All good scientists know that repeating an experiment is good for making sure your results are statistically relevant, so do it again. And again. Eat some chocolate. Have Fun!



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    162 Discussions


    Question 1 year ago

    what is the theory behind the experiment?


    2 years ago

    Nice, but how does the way in which the microwave emits radiation allows us to measure the wavelength in this fashion?


    3 years ago


    3 years ago

    This came up in my AS Physics exam today! I thought it was pretty cool that something I'd seen on Instructables turned up in the exam.


    5 years ago on Introduction

    Hahahahaha, oh boy, brilliant. Looks like you had a heck of a lot of fun with that chocolate hey....

    French Hawk

    7 years ago on Step 5

    ohk i am going to try it....and will have a cup of hot choco after dat


    7 years ago on Introduction

    I dont see how a microwave produces light waves though? The wave speed may well be measured as you describe here, but you are measuring the speed of something that is not light. Cool experiment all the same.

    1 reply

    Reply 7 years ago on Introduction

    Microwave radiation is electromagnetic radiation, as well as visible light. The only difference is the frequency. And all electromagnetic waves travel at the speed of light. So it is actually the same speed measured.


    7 years ago on Introduction

    This is a great project,
    but it would be more accurate with a plate of tiny chocolate sprinkles :)
    I mean from your results your microwave is of by 0.5 GHz.
    Its an ingenious idea! Tnx!


    7 years ago on Step 5

    My son, the PhD Physicist, is doing this experiment with his kids this weekend. If they do it a lot, they can take an average. That's a lot of chocolate.

    An alternative: a neatly spaced array of M&Ms would give an indication of the hot spot pattern in the microwave. Science Fair, here we come!