measuring speed of light at home?

[makc]

I will just quote the guy from xkcd thread 8000:

put chocolate or cheese or something in a microwave with the rotating plate removed. cook for a few seconds until the chocolate/cheese is melted in some places. this gives you the wavelength of your microwaves. the frequency should be 2.5 GHz (from water absorption spectra, or teh internets) the speed of light is then the frequency times the wavelength

is this valid experiment? I mean, was lightspeed used in any way when they measured those 2.5 GHz?

[hstarbuck]

is this valid experiment? I mean, was lightspeed used in any way when they measured those 2.5 GHz?

All electromagnetic waves travel at c, the speed of light. This is 3*10⁸m/s in a vacuum and close enough to that in air. Microwaves are the shorter version of radio waves (or longer than infrared) and if you measured 12 cm your calculation would be right on. Does the microwave really melt stuff at 12 cm spots? Seems unlikely. I think it is a little bit better distribution than that. At least at 6cm (half wave lengths) or less because it gets reflected off of a special fan. I can't try this at my house--no microwave--but I can try it at work.

Uneven heating in microwaved food can be partly due to the uneven distribution of microwave energy inside the oven, and partly due to the different rates of energy absorption in different parts of the food. The first problem is reduced by a stirrer, a type of fan that reflects microwave energy to different parts of the oven as it rotates, or by a turntable or carousel that turns the food...

What was the distance between your melted spots? Try center to center. Is it a valid experiment? If it actually works, for middle school, I would say yes.

[Amir]

...this gives you the wavelength of your microwaves.

i cant understand this part. how does it give us the wavelength?!

[Chris Peterson]

i cant understand this part. how does it give us the wavelength?!

I think they are trying to establish a relationship based on the wavelength of the microwaves, and the spacing of hot spots in the food, which could indicate a standing wave pattern in the oven category. In practice, I don't see this working, however. Microwave ovens use special fans to break up the signal and avoid standing waves, and even without the fan there are likely to be other sorts of interference and harmonic effects creating hot and cool spots at various spacings.

[wonderboy]

I think this is silly. If he is right though, fair play, its one of those things that could win you a science fair award. He seems pretty certain on the idea. However, i want to see proof of this through a video from him of some sort before I believe him.


Paul

[makc]

why the hell do you need a video?? don't you have an oven at home? I myself may actually try that tomorrow, when I buy some cheese. I have nothing to melt there at the moment.

however, my concern here is that if c is used to calculate 2.5GHz value, then we are not actually measuring anything.

[alter-ego]

...however, my concern here is that if c is used to calculate 2.5GHz value, then we are not actually measuring anything.

Ditto. As mentioned, microwaves are designed to prevent standing waves, certainly fundamental ones, which are what you want to measure. My guess is the hot spots will only be slightly warmer, with an uncertain period.

I've heard you can check your microwave heating uniformity by filling it with marshmellows and running the microwave for just the right time . But I've also heard that's likely to lead to a disaster

However, back in the days of scanning CRT TVs and analog air-wave broadcasting, it was not uncommon to see ghost images on the screen. In the Seattle area, major hills were a good reflector and if you know the e-beam scanning rate across the TV (# of horizontal lines and frame rate) and measure the distance on the TV screen between the primary and ghost image, you could get a reasonable estimate for c. In my case, I used c to get a handle on which hill was the problem. At least this method was more of an independent test i.e. had nothing to do with a coupled parameter (frequency). Oh the memories...

With great equipment technology becoming more available to amateur astronomers, someday we might be able to "video" record a laser pointer from the corner cubes on the moon and measure the time delay ourselves! Ok, it's a ways off..