Physics307L:People/Giannini/Speed of Light

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Steve Koch 20:19, 21 December 2010 (EST):Good job with this lab and very nice data and result!



The Purpose of this experiment was to measure the speed of light, through the use of a Photo-Multiplier Tube (PMT) and a LED attached to a meter stick which we could slide in and out. We were to determine this speed by recording a start and stop signal, with a delay on when the stop signal was read by our TAC. These two signals our measured by our oscilloscope, which gives us a voltage we can turn into time by the relationship between time and voltage given by our TAC. This time, when the distance light has traveled is divided by it, will give us, we believe, the speed of light.


The speed of light we measured was 30.5 +/- 0.3 cm/ns. The currently accepted value is 29.97 cm/ns which is within 2 SEM of our value. This, along with the results that Brian Josey has obtained when using the exact same equipment as us, click here to see Brian's results, seems to suggest that there is a predominate systematic error in our measurements, as Brian also got that his value was near 2 SEM of the accepted value, Brian's determined value was 31.0 ± 0.5 cm/ns. A summary of what we did to achieve this measurement can be found here under procedure.

Procedure and Results

The steps we took to get our value can be found here under procedure.

Here is our data, along with the graphs we have made to view it.

View/Edit Spreadsheet

Our final result, which determined through Linear Regression, by the use of the LINEST function in Google Docs, was 30.5 +/- 0.3 cm/ns.

Conclusions and Error

From our results, it is clear that there is some sort of systematic bias present in our experiment. Possible sources of systematic error are time walk, the cables we used to attach all the components in our experiment, and possible increases optical path length or diffraction due to travel through the air. Although the wires are very long and can cause some kind of extra delay in the signals we receive, I do not believe that they are the main source of error for this experiment. I believe, although we were very careful in this respect, most of the error can be eliminated by having a computer rotate the PMT to eliminate our time walk, as a computer has far better abilities at returning to an initial value then we do, since we have to estimate the position. Any remaining systematic error can be reduced, or even eliminated, by introducing our PMT and LED into a vaccuum, while also finding a way to shorten the cables that are connected to our TAC.

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