[tzs]

In the introductory optics class I took in college, one day the professor said "Let's measure the speed of light!" and pulled out a ruler.

The class laughed.

He then set the ruler on the table. The ruler was one of those where the tick marks are raised, not merely printed on. The ruler was metal and reflected light well.

He then shone a laser at the ruler, so that the light bounced off and hit the blackboard. The lines on the ruler acted as a diffraction grating and a diffraction pattern was visible on the blackboard.

He marked the peaks with chalk, then went back to the ruler and used the ruler to measure the distance from where it had been to the blackboard. He then used the ruler to measure the distance between the marks he had made for the diffraction peaks.

From those distances, and the separation between the lines on the ruler, and the frequency of the laser he was using, it was a simple calculation to get the speed of light.

Or course, in a sense this is cheating, as you have to know the frequency of the light source, so he had to use that as a magic constant in his calculation.

[lutorm]

Yeah, there are many ways of doing it with wave interference phenomena, but that requires you to know the wavelength. Doing it entirely based on speed of propagation is much more tricky, because it comes down to measuring small time differences.

[aphyr]

I've done the direct propagation measurement. Took about 20 hours (design, build, runs, analysis, report) in a four-person team, to obtain a value of .97 c. Laser, beam restrictor, beamsplitter, rotating mirror, and a 40 meter hallway with a mirror at the end. As the photons are traveling from the rotating mirror down the hall and back the mirror rotates slightly. Measuring the angular displacement of the beam with respect to the rotation speed gives you c. We used a low-res linear CCD array and oscilliscope, but you could probably do it at home... maybe with a dSLR sensor with suitably high response. You'd need a measurement of the CCD pixel density, but that wouldn't be too hard to find online. Then just handling the time sync issues.

[capnrefsmmat]

You can actually do it with a pulsed laser (hook up a laser diode to a function generator), a stationary mirror, and a photovoltaic cell. Hook up the function generator and the photovoltaic cell to an oscilloscope, mount the mirror a suitably long distance away, and you can actually see the timing difference between laser pulse and detection at the photovoltaic cell. Vary the distance and you can subtract out the delays in detection and such.

I did this as an experiment in a junior physics lab last semester. It felt like cheating. All you needed was twenty feet of space.

[juiceandjuice]

This is the Foucalt method.

We just measured the distance with a microscope, a 4 meter setup with a mirror to double the angle. It took about 2 hours and the only electronics were the rotating mirror. Our error was within 1%.

You can do the same thing with a 1000RPM mirror and about 50 feet with a caliper and get with 3% I suppose, as long as you are lined up good with the rotating mirror.