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Measure The Speed Of Light With Your Microwave
maddmike writes "There is a very interesting article on About.com that shows how to measure the speed of light using your microwave to melt chocolate. "
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Summary of the method used in the article:
* Slightly melt chocolate chips in your microwave
* Measure distance between melted spots
* This gives you (half) the wavelength of your oven
* Multiply by the frequency of your oven, you get the speed of light
That's certainly interesting, but guess what? Many scientists have done better (and much more expensive) measures, so we already know the speed of light quite well.
What we might not know as well is the frequency of your oven. So I suggest you reverse the above formula, and you measure the frequency of your oven (not always printed on the back, as the article admits) this way.
microwave experiments and links
In a vacuum, they're the same - all electromagnetic radiation will travel at c - 299792458 m/s. In the inside of a microwave oven, typically filled with air, but in this case also a certain quantity of chocolate, both will be lower. However, the higher energy wave (microwave) won't be slowed as much as the lower energy wave (light). So in actual fact, he is measuring the speed of microwave radiation in air, which is neither the same as that of light in air, nor electromagnetic radiation in a vacuum (it'll be somewhere between the two).
HTH.
I did the rapidly rotating octagonal mirror experiment in college. I got 2x10^8 m/s which is pretty slow. Either the mirror speed was off, or between runs I might have jostled the paper on the wall with the pencil marks marking the beam position.
You can measure the wavelength of light pretty easily with a ruler. But it has to be one of those shiny metal rulers, and it has to have black millimeter marks. Shine your laser onto the black marks at a shallow angle, measure the positions of the diffraction spikes that are reflected onto the wall, and from that, calculating the wavelength is trivial. It works pretty well.
Second problem is that it is actually difficult to stop something rotating. You need an inverted soup bowel or something to give clearance over the turntable drive peg.
Ok, if I use enough clearance to prevent rotation I get about 1cm between hot spots.
See my journal, I write things there
If you google for "speed of light" you get "News: Measure the Speed Of Light With Your Microwave - SLashdot - 2 hours ago"
Complete with (incorrect) overusage of CAPS and everything.
This experiment has no place outside the elementary school classroom. In fact, I think it has no place even there, because this method will be so wildly inaccurate that kids will learn the wrong speed of light.
Is it a wonder education is going to hell? We keep coming up with stupid, irresponsible "hack" methods of science that teach people the WRONG thing because we're spending too much money service the national debt to afford decent educational tools. Of course, it doesn't help that the "Educational" price for scientific instruments is often 2 to 3 times more than the "corporate" price - companies sucking at the government teat, of course.
"Superintendent Chalmers, thank you for your request for purchase of a time-domain reflectometer for use in your science classroom. While we value the ability of your students to perform valid and accurate experiments in physics, we've read somewhere on the Internet that a microwave oven will do just as well. They're about $50 a walmart. Therefore, your request is denied. Besides, I need a new Lexus. Sincerely, School Board"
Go ahead, mod me down, you know I'm right.
A microwave oven is a resonant cavity, and the resonant frequencies for the modes (TE/TM) are given by
where A,B,C are the dimensions of the cavity and i,j,k are non-negative integers (not all zero) which specify the mode.
This experiment does not "measure" the speed of light. All this "experiment" does is tries to isolate out a specific mode (i = 2, j = k = 0) and verifies that the frequecy rating printed on the back of the oven corresponds to this mode (which is still a cool thing to do).
You see, the manufacturer already implicitly *used* the value of c above in designing the oven and calculating the value of the number printed on the back of it, so the "experiment" is not capable of making a (independent) measurement of c.
Lest you think I am nitpicking, this kind of problem plagues us physicists all the time!
Absolutely worthless to anyone who bought a microwave in the last 2 years because they switched to a slightly modulating, slightly moving frequency emmiter which makes sure that it heats all of the food as quickly as possible instead of little hot spots. So basically, it melts everything at once in a new microwave. At least in a "good" new microwave.
*There's Klingons on the starboard bow, scrape em off Jim!*
You didn't measure the speed of light, you measure a wavelength. Unless you can show that you had some way to confirm the frequency of the light source that is not dependent on knowing the speed of light, then when you looked up the frequency of the light source you were effectively looking up the speed of light and using it to determine the speed of light. No wonder your answer came out close!
I'm an American. I love this country and the freedoms that we used to have.
How can there be stable nodes in the electric field within the oven if the distance between the oven walls is not a whole multiples of a half-wavelegth? Aren't the dimensions of the cavity set so that multiple patterns of standing waves will co-exist, each with its own nodes?
1) The microwaves are lower energy waves than light. There's just "more" of them in that cavity than visible light from the appliance bulb.
2) Generally speaking, materials (esp. un-ionized gas) will have a lower refractive index as frequency decreases. Hence, microwaves will be slowed less so than visible light in the air cavity of the microwave.
3) The patterns formed in the chocolate are due to standing waves set up in the cavity. The chocolate is a thin layer at the bottom, so the nodes will not reflect standing waves set up INSIDE the chocolate, because the wavelength is too large. So the patterns reflect the wavelength of microwaves in air, which is negligably less than the speed of light in a vacuum.
Measurement error from the ruler is a much larger (orders of magnitude larger) factor here.
Fuck Beta. Fuck Dice
Also, you can slightly change the frequency of a magnetron by varying the power supply. The resonant frequency of the cavities is a starting point, but the output can be "pushed" a little above or below it.
The power supply in a domestic microwave oven is designed to be cheap, not to be stable. It varies all by itself. So does the output frequency.
Sometimes ham radio operators will try to use microwave oven magnetrons as transmitter components. Hams have a spectrum allocation around 2.4 GHz, and it's tempting to think about getting high power cheap. One problem is that the output frequency jumps and drifts so much that you need to add circuitry to meet legal standards for a clean signal. We're talking fractions of a percent, close enough for measuring chocolate with a ruler but far too bad for radio communications.
Yes, and pulsing does make things even worse. The magnetron will behave a little differently during startup transients.
Oh, and don't play with magnetrons unless you can do it without killing yourself. The power supplies are immediately lethal and the kilowatt of RF isn't good for you either.
"There's more to science than mistreating animals. But frankly it's the part I like best." -- Dilbert
If you want better precision than chocolate or ants provide, you could print a grid onto thermal fax paper, moisten it so it won't catch fire, and put that into the microwave oven. See http://www.amasci.com/weird/microexp.html#demo. The idea is from JE Slone.