Home experiments to derive the speed of light?

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.

Simply, you would send out a radio signal, bounce it off a target and measure the time between when you sent the signal and when it was received. That delay is inversely proportional to the speed of light. Take a bunch of samples to integrate away short term clock inaccuracies (but don't integrate too long, because some types of clock noise will make your answer worse and not better) and you'll get a pretty good estimate of c.

Another GNU Radio based experiment would be to attempt to pick up multipath reflections of TV or radio broadcasts off large geographical features or buildings. Basically, you would capture two signals - a direct version of the broadcast and a delayed version. The errors would be large, but you would be able to get fairly close to a reasonable estimate of c (in air).

Before doing either of these experiments, mocking them up with a speaker and microphone or two ultrasound transceivers is a good idea and will save you a bunch of time. 40kHz is easier to work with than 2.4Ghz for sure.

It's similar to tzs' method below, in that you have to know the frequency of the microwave's radiation, so it's not entirely satisfactory.

But, given that, it's a really nice demonstration of standing waves with a cool result to calculate.

The method is to cut a bunch of marshmallows in to small chunks (or just buy mini-marshmallows), spread them on a tray in a microwave and turn it on. After a while, you'll see that the marshmallows are bubbling up in some spots, and not cooking very much at all in others. Measuring the distance between either the lows or the highs gives you the wavelength of the standing wave.

Given that wavelength and the frequency of the microwave radiation, you can calculate the speed of light.

I'm going to punt on it and leave the calculation up to the reader, mostly because I haven't thought about this in 15 years or so and I'm sure I'd miss a factor of 2 somewhere:).

Also, this will not work unless you can get a microwave where you can turn the turntable (the rotating plate thingy) off. Most microwaves didn't have a turntable in the early 90s, but this isn't true any more.

It should be an interesting and cheap way to measure the speed of light.