# The Naked Scientists Forum

### Author Topic: How to Measure the Speed of Light with a Bar of Chocolate  (Read 5971 times)

#### BenVitale

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##### How to Measure the Speed of Light with a Bar of Chocolate
« on: 01/11/2007 23:53:48 »
Science and chocolate... can there be a better combination....

Quote taken for Times On Line. The link's at the bottom if you want to see the other stuff

I'm open to suggestions on any other fascinating home experiments. In fact, it could be rather a good thread....

HOT CHOCOLATE

Is it true that you can measure the speed of light using nothing more than a chocolate bar and a microwave oven? The answer is yes. This is an astounding experiment that actually allows you to measure one of the fundamentals of science in your own home.

What do I need? A bar of chocolate (the longer the better). A metric rule. A microwave oven.

What do I do? Remove the turntable from your microwave oven – the bar of chocolate needs to be stationary. Put the chocolate in the oven and cook at high power until it starts to melt in two or three spots. This usually takes about 40 seconds. You should stop after 60 seconds maximum for safety.

What will I see? Because the chocolate is not rotating, the microwaves are not evenly distributed throughout the bar and spots of chocolate will begin to melt in the high-intensity areas, or “hotspots”. Remove the bar from the oven and measure the distance between adjacent globs of melted chocolate.

What’s going on? The frequency of the microwaves is the key. A standard oven will have a frequency of 2.45 gigahertz (the figure should be on the back of the oven or in the user manual). If your oven is 2.45GHz, the microwaves oscillate 2,450,000,000 times a second (you can adjust this figure depending on your oven). Microwaves are a form of electromagnetic radiation and therefore travel at the speed of light. If you know the frequency of the microwaves, finding out their wavelength will help you to calculate how fast they are travelling.

This is where the chocolate comes in. The distance between the globs of molten chocolate is half the wavelength of the microwaves in your oven, so double the measurement you have taken of the gap between the molten globs to find the microwave wavelength. In the New Scientist microwave oven the distance between the globs of molten chocolate was 6cm, so the wavelength in our 2.45 GHz oven is 12cm. To calculate the speed of light in centimetres a second you need to multiply this wavelength by the frequency of the microwaves: 12x2,450,000,000 = 29,400, 000,000, which is near to the true speed of light of 29,979,245,800cm a second (or 299,792,458 m per second).

Try it yourself, measuring as accurately as possible to get a figure even nearer to the true speed. If your chocolate bar is chilled beforehand, the molten areas tend to be more distinct when they first appear. You may find different chocolate bars, all of which taste delicious slightly melted, will aid your research. True scientists know that it is always important to double-check results.

PS: The hotspots – and consequent cold spots – that occur in ovens thanks to the wavelength of microwaves are the reason why ants can survive unscathed and uncooked inside a switched-on oven. They immediately scurry to the cooler areas and ride out the microwave storm.

#### JimBob

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##### How to Measure the Speed of Light with a Bar of Chocolate
« Reply #1 on: 02/11/2007 01:26:36 »
I will not waste chocolate!

#### DoctorBeaver

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##### How to Measure the Speed of Light with a Bar of Chocolate
« Reply #2 on: 02/11/2007 07:55:15 »
When I tried it, the fruit & nuts exploded

#### DoctorBeaver

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##### How to Measure the Speed of Light with a Bar of Chocolate
« Reply #3 on: 02/11/2007 07:55:53 »
Oooh... would it work with the Milky Bar Kid?

#### BenV

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##### How to Measure the Speed of Light with a Bar of Chocolate
« Reply #4 on: 02/11/2007 11:42:59 »
Worried about wasting chocolate?  Try it the Naked Scientists way!

http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/measuring-the-speed-of-light/

We did the very same experiment (months before the New Scientist book or the times article, I might add) but instead of wasting chocolate (and making a mess of your microwave) we did it using bread and butter. As long as you butter the bread right to the edge, you get a 'carpet' of butter, and the hot spots are seen where the butter melts.

#### Alandriel

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##### How to Measure the Speed of Light with a Bar of Chocolate
« Reply #5 on: 02/11/2007 14:40:15 »
Science and chocolate... can there be a better combination....

..........totally my kind of thing

even if it does involve *evil* microwave ovens

I kid of course

thanks for this Ben - SUPER !!!!

#### daveshorts

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##### How to Measure the Speed of Light with a Bar of Chocolate
« Reply #6 on: 02/11/2007 16:03:43 »
Also our explanation for what is happening makes some attempt to explain what is going on and doesn't have diagrams that are entirely misleading...

#### techmind

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##### How to Measure the Speed of Light with a Bar of Chocolate
« Reply #7 on: 02/11/2007 21:38:08 »
Also our explanation for what is happening makes some attempt to explain what is going on and doesn't have diagrams that are entirely misleading...
Your principle is good on the concept, but in the real world surely the standing waves will be 3D, not purely one-dimensional...?
Would you not also get interference from waves at other angles which could result in hotspots separated by a distance somewhat shorter than the true wavelength?
I know a bit about radiowaves and microwaves, but don't claim any specific expertise on the design of microwave oven caveties!

#### daveshorts

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##### How to Measure the Speed of Light with a Bar of Chocolate
« Reply #8 on: 03/11/2007 01:45:13 »
yes you will get some interesting waveguide effects slowing the speed of propagationdown a bit but it is good to considerably better than a factor of 2 which isn't bad.

#### lyner

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##### How to Measure the Speed of Light with a Bar of Chocolate
« Reply #9 on: 04/11/2007 00:01:32 »
Let's face it- all you are doing is measuring a distance ( the wavelength) and a time (one over the frequency). With those two, you can find the speed.
Chocolate is a tastier way of doing it.  btw, the method I have heard of is to use ice cream, instead.
In both cases, the answer is the speed of light (em waves) in the substance - not in space.

#### The Naked Scientists Forum

##### How to Measure the Speed of Light with a Bar of Chocolate
« Reply #9 on: 04/11/2007 00:01:32 »