# Fizeau's method

1. Jan 22, 2014

### haha1234

I don't know what is the principle of Fizeau's method.
In the experiment a rotating toothed wheel is used.But why the toothed wheel is used?
If the toothed wheel is not used,why the time for the light traveled cannot be measured?

2. Jan 22, 2014

### maajdl

The teeth are there to chop the light beam at a certain (high) frequency.
During the (small) time the light beam goes to the mirror and comes back, the teeth are rotating a (little) bit.
Therefore, the light pulse can either come back on a tooth or in between two teeth.
The light beam will then be respectively hidden or visible.
That's the clever trick Fizeau imagined to measure the speed of light.
A very nice experiment.
I would really appreciate to try it.
Must not be so easy as it sounds.

see http://en.wikipedia.org/wiki/Fizeau–Foucault_apparatus fig 2 for example.

3. Jan 22, 2014

### haha1234

But why the toothed wheel needs to be set up?
Why we cannot just measure the time the light traveled?

4. Jan 22, 2014

### AlephZero

If you shine a continuous beam of light, how would you know when the "same" bit of light has reached a different place?

The toothed wheel is just a mechanical way to "switch the light on and off" quickly, at a known frequency (you know the number of teeth, and you can measure the RPM of the wheel).

Today, you could use something like a laser to make short light pulses instead of a toothed wheel, but Fizeau and Foucault didn't have that technology available.

5. Jan 22, 2014

### voko

I would add to that that generating short light pulses solves only half the problem. The other half is detecting and timing them accurately. The cogwheel does both.

The interesting thing here is how the cogwheel was set in motion, how the velocity was controlled and stabilized. That must have been hellishly difficult in 1849. Too bad modern books just mention this experiment in passing.

6. Jan 22, 2014

### maajdl

Good remarks voko!
Would you know if there were some pedagogical remakes of this terrific experiment?

I stress this was a terrific experiment.
Imagine that nobody knew about the speed of light and that the most probable hypothesis at that time would have been an infinite speed.
Seeing the occultation and visibility of light as a function of speed must have been a very exciting fact at that time!

7. Jan 22, 2014

### AlephZero

One of the first measurements that showed the soeed of light was finite was observations of the orbits of Jupiter's moons. Given simple telescopes and clocks, and Newton's theory of gravitation, there were time discrepancies of about 15 minutes between the theoretical model and the observations, depending on the distance between the earth and Jupiter. Conclusion: light takes about 15 minutes to travel the diameter of the earth's orbit around the sun.

8. Jan 22, 2014

### maajdl

Right AlephZero!

9. Jan 22, 2014

### voko

This might be useful.

http://skullsinthestars.com/2008/03/31/fizeaus-experiment-the-original-paper/

It is probably going to be difficult to obtain the principal part of the setup, the wheel. It was 12 cm in diameter with 720 teeth. Even these days this is in the realm of high precision machinery.

10. Jan 22, 2014

### sophiecentaur

@WannabeFeynman
I Googled Image Formation and there were so many hits. Have you done the same? The reason I ask is that it is much easier to describe and explain with the aid of diagrams. Alternatively, look at a text book???. Someone (many people) have already drawn all those diagrams - so why not look at them and then come back with one or two specific questions?
I think you are expecting to get too much from just one post to PF.