The Speed of Light

Foucault Method

Daniel Ramirez April 11 2011

We have used the Foucault Method to measure the speed of light. Our best value calculated from the collected data is

The accepted value of the speed of light as defined by the 2006 CODATA recommended value is

The error in our value places our calculated result within two standard deviations of the accepted value.

Abstract

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c = (3.05 ± 0.3)x108 ms

History I

•  Light traveled from eyes to object then back to eyes

•  If stars appeared when eyes were opened, then the speed of light must be infinite.

The ancient Greek, Heron of Alexandria argued:

History II

•  The idea of an infinite speed of light lasted for centuries

•  1729 - James Bradley used starlight aberration to get the first accepted measurement of c

Starlight Aberration

Due to the velocity of the earth, light appears to arrive at an angle.

The Foucault Method

•  1862 - Foucault’s method for measuring the speed of light directed a beam of light at a rotating mirror.

Experimental Setup

Experimental Method

•  Optical alignment is the true challenge of the experiment

•  Data collection is straightforward. Increase the speed of the rotating mirror to

~1500 rpm cw and record the displacement of S with the micrometer. Repeat the measurement using

~1500 rpm ccw.

Data

c Calculated for 10 runs

Discussion

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c = (3.05 ± 0.03)x108 ms

Our calculated value of c for 10 runs was

With 2% relative uncertainty

This places our value within two standard deviations of the accepted value.

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c2006CODATA = 2.99792458x108 ms

Conclusion •  We have tested the Foucault method for reliability in measuring the speed of

light.

•  Our result of

successfully demonstrates that this method is an adequate means to determine the speed of light.

•  For this particular experiment, the accuracy and precision could be refined by increasing the length, D, as well as by increasing the speed, ω, of the rotating mirror. –  High quality laser, optically processed to reduced aberrations –  D= 1 km –  ω=20,000 rpm

-with these parameters a of ~2cm is possible

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c = (3.05 ± 0.03)x108 ms

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