Home experiments to derive the speed of light?

http://physics.stackexchange.com/q/357/5450

by sethjrogers • 15 years ago

54 13 15 years ago

physics.stackexchange.com

Interesting answers there. I remember doing the capacitor experiment in a first-year physics course in university. Though we were measuring mu sub 0, the permeability of free space. The formula is just reversed though.

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

tzs• 15 years ago

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• 15 years ago

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• 15 years ago

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.

mjb2• 15 years ago

Starting with something like a GNU Radio board (http://gnuradio.org/) and a commercial antenna, you could put together a basic CW or ICW radar in a weekend without too much trouble. A little bit of care is required to not break any local laws or piss people off.

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.

spatten• 15 years ago

I've done it with a microwave and some marshmallows (I taught this lab to some first year college students long ago when I spent a term as a college physics instructor).

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.

mikeryan• 15 years ago

This is covered in the link only it uses a chocolate bar instead of marshmallows.

EDIT: http://www.youtube.com/watch?v=9O2Keu6o3i0

chemmail• 15 years ago

Wouldn't the face that you cannot achieve a perfect vacuum make measuring the true speed of light to exactly c impossible?

mjb2• 15 years ago

Not really, no. The speed of light in air is very nearly identical to the speed of light in a vacuum. Wikipedia gives the refractive index of air as 1.0003, which is the ratio between the speed of light in a vacuum and the speed of light in air.

The use of air as a medium rather than a vacuum is unlikely to be a significant source of error in an amateur measurement of c.

wmobit• 15 years ago

You don't use experiments to 'derive' the speed of light. You use Maxwell's equations.

Confusion• 15 years ago

Well, if you're going that way, I'd say you can't derive it at all, because the speed of light is a defined constant.

Home experiments to derive the speed of light?

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