Leftover Valentine’s Chocolate? Use It to Measure the Speed of Light
-
By Kathy Ceceri
- February 15, 2010 |
- 8:00 am |
- Categories: Projects and Activities, Science and Education
You can use Valentine's Day chocolate and your microwave to perform sophisticated physics calculations! All images: Kathy Ceceri
If you’re a long-time reader, you may remember the great leftover Easter Peeps microwave experiment. Well, today we’re going to be nuking leftover Valentine’s Day chocolate to demonstrate one of the constants of physics, the speed of light. Chocolate makes a very appropriate medium, because the heating property of microwaves was first discovered by a scientist whose candy bar melted in his pocket when he got too close to a microwave device being tested for use in radar.
WARNING: This experiment may take several tries to get right. We are not responsible for any weight gained. To avoid familial strife, be sure to only do this experiment with your own chocolates or with candy which you have been authorized to access. You can probably find some leftover boxes on sale this week.
The demonstration works because microwave ovens produce standing waves — waves that move “up” and “down” in place, instead of rolling forward like waves in the ocean. Microwave radiation falls into the radio section of the electromagnetic spectrum. Most ovens produce waves with a frequency of 2,450 megahertz (millions of cycles per second). The oven is designed to be just the right size to cause the microwaves to reflect off the walls so that the peaks and valleys line up perfectly, creating “hot spots” (actually, lines of heat).
What you do with the candy is to find the hot spots and measure the distance between them. From that information, you can determine the wavelength. And when you multiply the wavelength by the frequency, you get the speed! Here’s what you do:
- Make sure the candy is in a microwave-proof box. Better yet, take the chocolate out and put in a microwave safe dish.
- Remove the turntable in your oven. (You want the candy to stay still while you heat it.) Put an upside-down plate over the turning-thingy, and place your dish of candy on top.
- Heat on high about 20 seconds.
- Take the chocolate out and look for hot spots. Depending on the candy you use, you may have to feel the candy to see where it has softened. With the cherry cordials we used, we saw several shiny spots and one place where the chocolate shell melted through, releasing the sweet syrup inside.
Measure the distance between two adjacent spots. This should be the distance between the peak and the valley (crest and trough) of the wave. Since the wavelength is the distance between two crests, multiply by 2. Finally, multiply that result by the frequency expressed in hertz or 2,450,000,000 (2.45 X 109 for my son who is just learning scientific notation).
In our trial, we measured a distance of roughly 6 centimeters. 6 x 2 x 2,450,000,000 = 29,400,000,000 centimeters per second, or 294,000,000 meters per second. This is awfully close to 299,792,458 meters per second, which is the speed of light. Not bad for some leftover chocolate and a kitchen appliance!
I discovered this experiment at Null Hypothesis, although it can be found all over the Internet, including many versions with fancy charts and animations. By the way, melted chocolate bars are perfect as ice cream topping. Just saying.
Kathy Ceceri also blogs at Home Physics.
Gotta love how physics scale perfectly.
Why did you use 2450 Mhz as the frequency, rather than 2500 Mhz mentioned in the beginning of the story?
Blech…I hate cherry cordials! What a waste of perfectly good
chocolate.
@goose: I think you have to take the microwave frequency from the owners manual, since the frequency might change depending the brand and model of the microwave oven, the 2.5Ghz is just an aprox. measure look for it on the null-hypothesis site
Uber-geeky Crazy Aaron of Thinking Putty fame shows how to do this with thermochromic putty on his site (http://www.puttyworld.com/usethputofis.html). It’s not as tasty as Valentine chocolate but it’s also not as fattening.
How curious that your 294Mm per second in your measurements, precisely the same as in the Null Hypothesis article, that you also just happened to use 2.45ghz instead of the earlier mentioned 2.5ghz sure have to be a coincidence.
So the question is, is the speed of light 294Mm/s or were you feeling sloppy and just copy pasted from the null hypothesis article?
I’d love to give this a go, but there’s no such thing as ‘leftover’ chocolate in my house!
@goose Thanks for the catch. That’s what I get for using two different sources of info. The manual for my own microwave says the frequency is 2,450 MHz.
@pkblaster The important measurement is the distance between hot spots. As long as you get a number around 6 cm, you’ll get the same result as Null Hypothesis. Although pinpointing the center of a soft spot on a piece of chocolate is not an exact science, I thought this experiment was too cool to pass up.
I think I’ll take your word for it, instead of melting perfectly good chocolate. In case there is any chocolate left.
That’s a very cool experiment. I think I might like to try something similar with something like that black material that changes colour with temperature. Not as easy to find as chocolate, but I’d like to find a way to see the standing wave pattern more clearly. Oh, and pkblaster needs to chill out.
What is missing is a simple belivable explanations of WHY this is the speed of light, especially since no light is used.
NO! Don’t tell me formulas and definitions!
But explain to a 10-year old…Speed of microwave, maybe. Then why microwave speed and light speed might be the same.
This works very well with a plate of marshmallows too and since there is a visible burn spot if you leave the oven on long enough, it can be easier to measure as well.
Both light and microwaves are forms of electromagnetic radiation, occupying different parts of the spectrum. All travel at the same speed (speed of light, for short). In order of increasing frequency and decreasing wavelength: radio waves, microwaves, terahertz radiation, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays. http://en.wikipedia.org/wiki/Electromagnetic_radiation
Milk chocolate is crap, once you’ve had pure cacao.
If you take into consideration that the speed of microwave radiation is slightly slower in air than in free space your experiment is even more accurate.
this got me hungry…
I don’t know anything about microwave technology, except that Hot Wheels catch fire when my son starts the microwave with one inside it. But I do know that this experiment assumes that the waveform of microwave radiation has only two peaks (”crest and trough”) as described in the article. While a large class of harmonic signals fits this requirement there is a much larger class of harmonic signals that do not. While microwave technology may very well be able to create precise single sinusoids at such high frequencies without harmonic distortion, can anyone here corroborate that or not?
Just read a little more on microwave generation on Wikipedia. They indicate that a Cavity Magnetron (http://en.wikipedia.org/wiki/Cavity_magnetron) is typically used to generate microwave radiation in microwave ovens. They indicate that frequency is not precisely controllable with such a device but they do not provide any details. Also, since the cavity of the device resonates the microwave signal to produce a desired frequency, the result signal will decidedly be a complex harmonic signal — which may or may not have two distinguishable peaks dependent on the energy distribution of the harmonics and their phases.
I understand fully all the physics concepts mentioned in the article, but there is one thing that I just cannot wrap my head around and that is the entity termed “leftover chocolate”. Any scientists out there that can explain that to us (in simple terms, please!)?
“While microwave technology may very well be able to create precise single sinusoids at such high frequencies without harmonic distortion, can anyone here corroborate that or not?” Magnetrons put out a pretty noisy waveform but it’s centered around 2450, which is also where your wifi works. The harmonics are at multiples of this frequency but aren’t very strong. You will notice hot spots, but these are maxima of the standing wave where the voltage is highest. The voltage gradually subsides to minima where the current is highest. So you’ll notice a broad change of temperature.