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

Picture of 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!

Picture of 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!!!

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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

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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

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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!


jeff.90g (author)2017-03-12

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

zZcube (author)2016-05-24

notforfooduse (author)2016-05-24

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.

Mariska Botha (author)2013-09-04


BunnyRoger (author)2013-09-03

Very very interesting mr bradpowers.

Amanda Culbert (author)2013-09-02

Looks so delicious! Big chocolate fan here.

MAApleton (author)2013-08-29

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

code_neon (author)2012-06-23


French Hawk (author)2012-03-25

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

waynemov (author)2012-01-07

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.

6stygos (author)waynemov2012-02-01

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.

Tkdwn (author)2011-11-09

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!

tpobrienjr (author)2011-07-03

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!

bradpowers (author)2010-06-22

Did you do the experiment?

bradpowers (author)2010-06-22

Why thank you!

FireCrazyAlchemist (author)2008-06-17

could you do this with white chocolate?

I don't see why not?



adlabens (author)bradpowers2008-06-18

Actually, the answer to the question of whether the experiment can be done with "white chocolate" is a resounding "ABSOLUTELY NOT" - because there's none of the chocolate liquor in white chocolate. It really should be called white "candy" as there's none of the essential component of chocolate in white chocolate. No part of white chocolate is actually chocolate - & it's also not very white, either!!! But, you'll probably get more accurate results using Dark Chocolate - because the darker the item being microwaved, the more it absorbs & retains the heat. That's why you don't microwave popcorn in a brown paper bag (it'll burst into flames), but it's ok to microwave popcorn in a white paper bag.

seventhc (author)adlabens2008-06-19

I actually use normal popping kernels in a regular brown paper lunch bag (folded over a few times) in the microwave, no flames..healthier corn, much cheaper.

adlabens (author)seventhc2008-06-19

Seriously, I don't think the color has anything to do with it. I think the content of the brown paper is different (density, recycle, etc.) from the white paper bags. All I know is that we sold hundreds of microwave ovens every year for 25 years, and every single owners' manual said not to pop popcorn in brown paper bags. I follow this advice, personally, but you can definitely do what you want. The owners' manuals may say that simply for liability protection purposes. I don't really know. But, they said it. Mom & Dad closed the store in 1999, and I've only bought one since then for myself (which also said it), so things may have changed.

killerjackalope (author)adlabens2008-06-24

It's not because of colour, now popcorn bags are brown but the reason has to do with the composition of brown paper, at a guess I'd say the fibres are still present to a great degree and would have a little water in them...

you're right - its nothing to do with colour. the reason that you shouldn't microwave popcorn in some brown paper bags is the plastics in the paper (thats right - they're part plastic) have a similar absorbsion spectrum to water and so are heated by the mircrowaves in the same way that water is. this can make it leech into food and although it wont kill you straight away may have detramental effects after prolonged use. same goes for some plastic containers - their absorbtion spectrum is the same as water, so they absorb the em waves as heat energy and they melt.

bradpowers (author)henrywfstone2010-06-22

Your first problem is that you're eating popcorn and not melty chocolate.

Aye, though once you see the difference in safe and unsafe plastics it's pretty easy to tell by look and feel, though tupperware and microwaves were never meant to be anyway... Interestingly though dark chocolate would be the best for other reasons, which could be the issue, more likely though people have gotten confused when it comes to the emission and absorption of infra-red radiations... I can't remember but UV is in the same category being part of the light spectrum, ugh physics was some time ago...

white chocolate has a higher fat content compared to dark chocolate and fat contains water. this makes it melt easier so must have something to do with it.

yeah, that's true but because dark chocolate has a higher melting point it gives you the most accurate result as there isn't as much melting in areas not being directly hit, really though it would also look the coolest afterwards... With a high power microwave with the right wavelength you could cut the chocolate up that way...

peterthehun (author)adlabens2008-06-19

technically, considering the heating source is a relatively narrow and non-visible wavelength, I don't think the 'dark' appearance of Dark Chocolate necessarily counts in this way. Generally a darker and greener looking material would be more likely to have high absorption near the red end of the spectrum which is closer to IR and microwaves etc... just think, microwaves are tuned to a wavelength of high molecular resonance for water and thus very high absorption, despite the fact that water is highly transparent to visible wavelengths!

bradpowers (author)peterthehun2010-06-22

mmmmmm. Green Chocolate.

fentanyl3 (author)adlabens2008-06-21

color has nothing to do with how well an object absorbs the energy of invisible rf waves, the water content is what you need. 2.4 GHZ is attenuated very well by water, which is why 2.4 GHZ is used as the operating frequency for microwaves

Though colour means nothing dark chocolate will get better results, the reason being is it has a higher melting temperature, seems counter intuitive right? The reason is because you get less 'melt around' so the measurements would be accurate...

Except dark chocolate is gross, and this isn't about accuracy.

Superhobo40 (author)bradpowers2009-10-21

amen to that bradpowers

yeah but it was a weird argument and I thought I'd give them the answer... I love dark chocolate, bittersweet, not pure bitter though...

WTF!!!!!!!!!! This i was only askin cuz i dont like brown or dark chocalate and this should be a big flippin agrument

Lol I know but I thought I'd weigh in and give a real answer... Especially since people were talking rubbish, that and I like chocolate...

Jerk lol

Superhobo40 (author)fentanyl32009-10-21

2.4 GHZ +/- e

smurfsahoy (author)fentanyl32009-03-19

In some sense, color does matter. Materials that absorb different rf waves of any type could be said to vary in color, though not visible color in this case. Flowers have distinct color patterns in UV light for pollinating insects to see, etc. Is that really so different from saying that water is a dark color at 2.4 GHz?

jmadray (author)fentanyl32008-09-01

Just curious but since microwaves operate at 2.4 ghz, is that why you sometimes get interference on cordless phones operating at the same frequency?

fentanyl3 (author)jmadray2008-09-02

Possibly, but more likely the interference is coming from the front end of the microwave, meaning that the interference is coming from the drive circuitry vs the resonator, unless the resonator is poorly shielded, and out of spec, in that case it could overwhelm a little 1 watt transmitter very easily depending on the type of cordless it is. Most likely that the interference is coming from front end noise through the power lines inside the house to the phone, IE interference leaves the microwave through the power cord, and then interferes with the phone downstream

Llamarama (author)adlabens2008-10-11

Just for the record, white chocolate is cocoa butter. An essential part of the chocolate. Without it you would have a handfull of cocoa powder.

bradpowers (author)adlabens2008-06-19

White chocolate should melt right? That's all that matters.

henrywfstone (author)bradpowers2008-09-02

yes... and easier than dark chocolate as white "chocolate" is really just cocoa fat and sugar.

Antpopper (author)adlabens2008-06-19

Actually, "Newman's Own" popcorn comes in a brown paper bag, and it hasn't burst into flames yet.

adlabens (author)Antpopper2008-06-19

If you say it is, then I'm sure it's true. However, the microwave ovens I've bought for use in my own homes, and the ones I used to sell, all had owners' manuals that said not to use a brown paper bag to pop the popcorn, but that a white one was ok. Probably "Newman's Own" has a special formulation on the bag. But, the regular, run-of-the-mill brown paper bags were a "no-no" according to the owner's manuals on GE, Hotpoint, Whirlpool, Thermador, Jenn-Air, and a couple of other brands we used to sell (before Mom & Dad closed the store & retired).

KD7WHQ (author)adlabens2008-06-19

The ONLY difference between white and brown paper bags is the bleaching process the white ones go through while roll paper. There is no difference whatsoever otherwise, and definitely at the microwave frequencies of RF. Yup, Radio Frequency..

adlabens (author)KD7WHQ2008-06-19

You may be exactly right, tho the texture is definitely different. Maybe because of the bleaching process, but the white bags (that we get at the grocery store here) are a smoother texture than the brown ones.

008 (author)adlabens2008-06-19

Did it specifically say you could use white bags? If not, then the manual is likely assuming all paper bags are brown. Perhaps they just don't want you popping corn in "unofficial" packaging due to liability concerns.

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