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Measuring the Speed of Light

by hy23
Tags: light, measuring, speed
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Jan27-11, 12:40 AM
P: 64
If we were to measure the speed of light as the scientists in the 1800s did (such as Foucault and Fizeau), without knowing the relationship between speed of light and its wavelength and frequency, would it be possible to do so using only simple apparatuses like mirrors and low powered motors and lasers that one can find around the house or buy at a nominal price?

By the way, Fizeau's method involved having a light beam from a light source go between the teeth of a rotating toothed wheel, the beam hits a mirror some 8km away, comes back and goes between the same teeth of the rotating toothed wheel. Then he can test how much he has to speed up the wheel so that the returning beam gets blocked by the teeth, and how much more he has to speed up the wheel so that the returning beam goes between the next two teeth (the slit beside the slit it passed thru initially).

The problem with this experiment is the alignment of the mirror has to be perfectly perpendicular to the light beam, the motor has to spin really fast, possibly at 10000 rpm, and how can one detect the returning beam (it seems to me that the returning beam will just collide with the incident beam).

Is there a possible alternative to this type of experiment that has less complications?
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Jan27-11, 01:11 AM
P: 3,387
Well so far as that experiment goes, you could align the light source at an angle. The beam would go through the first slit and travel to a mirror which it would hit at just off normal and return to the device.

This angle would mean that both beams are separate.

If the wheel wasn't spinning, you would observe the light leave one tooth and it's return path would mean it comes back through the tooth next to it. Giving you entry and exit points that are distinct.

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