Hi Jim:
Check out:
http://www.ted.com/talks/lang/eng/clifford_stoll_on_everything.html
Here's part of an email from Clifford:
"Oh, the speed of light?
For detectors, I use two fast-response photodiodes and feed their outputs into
opamps.
The experiment uses one cheap laserpointer (they cost a buck or two on ebay - I buy 25 at once, and then select the
ones with the fastest switching rate.)
The laser pointer is fed by a signal generator (I use an HP 3312A); the square wave output connects to the laser
pointer with a pair of clipleads. I start with the laser switching about once a second, to show the kids that it really
is turning on and off. THen I boost the speed to a few hertz, then tens, and hundreds of hertz. Eventually, I get it
switching on/off at 2 to 5 MHz.
I aim this switched laser beam at a distant mirror (across the room - maybe 10 or 15 meters away). Coming back along
almost the same path, this long-path beam then hits a lens which focuses the beam onto one photodiode. The photodiode
feeds its pulses into an opamp, and then into the bottom trace of a dual trace 100MHz oscilloscope.
I then create a short-path beam using a beamsplitter that's right next to the laserpointer. I use a piece of
microscope slide cover-slide for the beamsplitter. A few inches from the beamsplitter I set the short-path photodiode,
which goes through the 2nd opamp and into the upper beam of the squigglescope.
There'll be a time delay in the arrival of the long-path beam; I get the students to measure this time difference.
Then we measure the path lenght difference with a tape measure, do a division, and out pops the local speed of light.
Lots of gotcha's ... for instance, I make sure that I trigger the oscilloscope on the output of the function
generator. Also, alignment is very difficult, and requires rock-solid furniture (seldom found in classrooms).
It's a fun project with high school students, because we spend a long time afterwards discussing errors and problems
in the system ... typically, we measure things about 20 or 30 percent off. The main source of error turns out to be
mismatched responses in the two photodiode/opamps.
Have Fun,
Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/Brooke4Congress.html
Jim Lux wrote:
While delayed, I would think that the signal freqs would still need to be maintained... hmmm, maybe not...
interesting science project... anyone? anyone? ;-)
Jerry
----
I'm waiting to see a good time-nuts project at the science fair. (at any level
up to ISEF)
There's a lot of good ones out there (perhaps not on the scale of tvb's experimental demonstration of gravitational
effects on atomic clocks) that would lend themselves to execution by everyone from 6th to 12th grade. Clearly, since
people do spend their entire professional life doing this and write dissertations on it, it can be up to ISEF or
Siemens Talent Search standards.
Maybe we could come up with a suggested list and start shopping it around.
Jim
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