Measuring the speed of light or radio waves without an oscilloscope

[resurgence2012]

Thanks - yes I looked at it carefully. Someone else just suggested a counter with a 30MHz clock connected to and two photogates. That does seem to me to be a very direct approach. So I have started off down that avenue. I am also trying to keep on mind the different technologies involved, and this strikes me as an avenue that will be relevant for my own students.

 

[resurgence2012]

Doggy, yes that was pretty much exactly what I was thinking. I have just been digging around on the net trying to find a fast counter which has about ten digits. Can you point me in the right direction?
Thanks very much Peter

 

[misterT]

http://search.digikey.com/us/en/products/HEF4017BP,652/568-3080-5-ND/1023564
i didnt get time to see if thats exactly wat he needs, that was just the first counter i pulled up
and again he will need to bounce that beam around several dozen times aswell

Oh, you were talking about a counter IC circuit. Building an accurate circuit would be a big challenge.. especially the detectors. The delays in the detector signals would have to be matched, and they would have to stay pretty constant.

 

[resurgence2012]

Yes - most roads seem to lead to another very steep learning curve - bit it is all good.

I am in Thailand and I found earlier a company that makes a six digit stop watch that looks like it is either being controlled by a PIC - it's a long shot but I think I might be able to find an electronics student or even a professional who might be interested in taking something like this and seeing if there is a way of speeding it up about 10,000 times. It currently measures down to 1/100 of a second.

It is just a case of finding the right person with right level of skill and enthusaiasm . Thailand is pretty good for things like this.

Then again I am close to accepting the errors of my ways and considering buying an oscilloscope. Ha ha

 

[misterT]

I am in Thailand and I found earlier a company that makes a six digit stop watch that looks like it is either being controlled by a PIC - it's a long shot but I think I might be able to find an electronics student or even a professional who might be interested in taking something like this and seeing if there is a way of speeding it up about 10,000 times. It currently measures down to 1/100 of a second.

Have you worked out the math? How long distance you can make the light travel and then how long would be the time-delay?

 

[resurgence2012]

That is a very good point. I have been working on the assumption that I can get my 2000 mW
green laser to travel 300 meters but I havn't actually tried it out prioperly. I think it might easily go ten times that distance which makes timing it a lot easier.

I may well go into school tomorrow and at least try to test out that part of the equation. Thanks for pointing that out. I am going try bouncing back and forth along the corridor, which is about forty meters, and see just how many times it can be reflected.

 

[unclejed613]

here's another way to do it....

Another way to measure the speed of light is to independently measure the frequency f and wavelength λ of an electromagnetic wave in vacuum. The value of c can then be found by using the relation c = fλ. One option is to measure the resonance frequency of a cavity resonator. If the dimensions of the resonance cavity are also known, these can be used determine the wavelength of the wave. In 1946, Louis Essen and A.C. Gordon-Smith establish the frequency for a variety of normal modes of microwaves of a microwave cavity of precisely known dimensions. The dimensions were established to an accuracy of about ±0.8 μm using gauges calibrated by interferometry.[95] As the wavelength of the modes was known from the geometry of the cavity and from electromagnetic theory, knowledge of the associated frequencies enabled a calculation of the speed of light.[95][97]

The Essen–Gordon-Smith result, 299,792±9 km/s, was substantially more precise than those found by optical techniques.[95] By 1950, repeated measurements by Essen established a result of 299,792.5±3.0 km/s.[98]

A household demonstration of this technique is possible, using a microwave oven and food such as marshmallows or margarine: if the turntable is removed so that the food does not move, it will cook the fastest at the antinodes (the points at which the wave amplitude is the greatest), where it will begin to melt. The distance between two such spots is half the wavelength of the microwaves; by measuring this distance and multiplying the wavelength by the microwave frequency (usually displayed on the back of the oven, typically 2450 MHz), the value of c can be calculated, "often with less than 5% error".[99][100]

 

[alec_t]

Aren't "mode-stirrers" used in domestic ovens precisely to avoid hot-spots?

 

[RichTheDude]

PIC timers can unofficially count upto 60 MHz (look up frequency counter projects). With a typical 18F PIC you could configure a timer as a 16 bit counter, this would allow you to measure 327 km round trip - somehow I doubt you will ever be measuring that!

Rough calculations 60 MHz would be ~5 metres resolution. You could pulse the laser with a monostable, and this trigger pulse (from the PIC) to the monostable could be used to SET a flip flop. The output of this flip flop could be tied to an AND gate, which gates a 60 MHz external clock into the Timer 0 Clock Pin.

A fast photodiode could have it's output discriminated (i.e. threshold comparator) and this could trigger the reset of the said flip flop - hence stopping the clock from passing the AND gate. This pulse could also be fed to an external interrupt pin of the PIC, so it knows when to read the counter value, reset it and wait for another measurement command (probably a button ). You could then send this counter value over serial or display it on alpha numeric LCD.

74ACT or 74F parts maybe upto this task (above about 25 or 30 MHz I would go for a CPLD personally) - should be cheap to experiment with such a system anyway

You will need a good collimated laser, and some collection optics of some description. A laser mounted coaxially on a Newtonian reflector telescope would probably be what I go for but is probably way overkill - trouble is with such a system you need a fairly long round trip for the error of such a crude instrument not to be significant. A tricky problem to do cheaply and easily.

 

[unclejed613]

Aren't "mode-stirrers" used in domestic ovens precisely to avoid hot-spots?

some ovens have them, some don't. usually if there's a turntable in the oven, there isn't a "stirrer". to keep costs down manufacturers rarely use both

 

[alec_t]

Right. All the ovens I've seen have had a turntable, so I guess no mode-stirrer. Now, where's that tub of margarine .......

 

[panic mode]

"Here is a Finnish version of the lab-assignment."

if you say so, it looked Navajo to me

 

[ccurtis]

Those laser tape measuring devices determine distance by assuming the speed of light as a given and measuring time. So, why not get a known distance and use the reading, again to measure time, and then calculate the speed of light. They are pretty cheap.