Physics307L:People/Barron/Final rough

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Speed of Light from a Cardboard Tube

Alexander T. J. Barron

Junior Lab, Department of Physics & Astronomy, University of New Mexico


Abstract

I measure the speed of light in air utilizing a time-to-amplitude converter (TAC), a photo-multiplier tube (PMT), and an LED pulse-generator in a light-tight environment. By positioning the pulse-generator at varying distances from the PMT/TAC apparatus, one can obtain various data sets corresponding to the change in distance between the two components. For each change in distance, I record a new amplitude from the TAC, corresponding to change in time readings. Plotting change in distance vs. change in time yields the perfect environment for taking a least-squares linear fit of the data, of which the slope is the speed of light in air. In the process of finding cair, I use different data-taking strategies as well as investigate a phenomenon known as "time walk," which without correction nullifies any useful data taking from this equipment set.

Introduction

All experiments measuring the speed of light involve taking measurements of time taken for light to traverse a given path[1]

References

  1. Dorsey, N. Ernest. "The Velocity of Light." Transactions of the American Philosophical Society, New Series, Vol. 34, No. 1 (Oct., 1944), pp. 1-110. American Philosophical Society. JSTOR

    [Dorsey-Transactions-1944]

    Dorsey covers tomes of material in this paper, including a nicely-put summary of error analysis and the least-squares approach. He analyzes a number of historical experiments measuring the speed of light and reviews their accuracy based on procedure, equipment, and effectiveness of error documentation. There doesn't seem to be any mention of confidence intervals in a standardized way, but rather each uncertainty is reported based on logical arguments and various extremum.

  2. Heyl, Paul R. "The Application of the Method of Least Squares." Science, New Series, Vol. 33, No. 859 (Jun. 16, 1911), pp. 932-933. American Association for the Advancement of Science. JSTOR

    [Heyl-Science-1911]

    Heyl argues that the method of least-squares to average out error may be flawed, specifically in the context of extended Michelson-Morley-like experiments. He proposes a "mathematical theorem," providing a rule of thumb regarding acceptable least-squares error analysis.

  3. Mulholland, J. Derral. The Speed of Light." Science, New Series, Vol. 180, No. 4093 (Jun. 29, 1973), pp. 1321-1322. American Association for the Advancement of Science. JSTOR

    [Mulholland-Science-1973]

    Mulholland argues for the light-second as the more fundamental and useful unit regarding the speed of light.

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