# Can you use a laser to measure the speed of light with a rotating mirror?

I have learned that the classical measurement of the speed of light with a rotating mirror does not work with a laser (as opposed to, say, a mercury-vapor lamp).

Can you tell me if and how coherency of light is necessary/contraindicated for the success of this experiment?

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When I wrote this question, I got the message: The question you're asking appears subjective and is likely to be closed. Is this just based on the grammar "Can you use"? Should I have used the title "Is it possible to" instead? – Phira May 5 '11 at 12:07
The rotating mirror method of Leon Foucault works in principle with any light point source. You are measuring an angle. – Qmechanic May 5 '11 at 13:09
I remember doing this in an undergraduate lab course. I think we used a laser, but it was a long time ago, so I could be wrong. I certainly can't see any reason it wouldn't work. – Ted Bunn May 5 '11 at 14:40
Get a stopwatch, a laser, and a mirror (a laser pointer will be fine). Stand back from the mirror and at the same exact time, hit the stopwatch and the laser pointer. The moment you see the laser in the mirror, hit the stopwatch again. – Sam May 5 '11 at 18:44
The "subjective" warning was triggered by your use of "you" in the title. – dmckee May 6 '11 at 18:28

Coherence of light doesn't harm this experiment because this experiment is based on geometric optics. However, laser is actually an immensely good source of light which has a very well-defined direction of the light rays. That's why it's much better to use a laser as the light source than the candles or whatever Foucault used to use.

To see a web page of people who have repeated the Foucault rotating mirror measurement of the speed of light using a He-Ne laser, see e.g.

http://www.pas.rochester.edu/~pavone/particle-www/teachers/demonstrations/FoucaultDemonstration.htm

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Candles ? Hmmm :=) At that time such a lab was located in a room with windows to south an with a Heloistat there. If sun was not to be used for some reason, Foucault had the possibility to use "limelight" a la Drummond. – Georg May 5 '11 at 16:46

Did the Wikipedia article specifically mention using a rotating mirror with laser light? When I was in undergrad we used a pulsed laser to measure the speed of light, but not with a rotating mirror. We split the beam and directed one one half at a photodiode. The other half was sent across a long room a few times with mirrors before hitting the photodiode. We used the timing measurement from the photodiode and manually measured the path length.

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You are right that the laser is only mentioned in the interferometry section. – Phira May 5 '11 at 13:21