User:David K. O'Hara/Notebook/physics 307 lab/speed of light summary

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Speed of Light Summary

SJK Incomplete Feedback Notice
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My feedback is incomplete on this page for two reasons. First, the value of the feedback to the students is low, given that the course is over. Second, I'm running out of time to finish grading!

The object of this lab was to calculate the speed of light, one of the fundamental physical constants of the universe. This was accomplished by shining an LED down a dark cardboard tube onto a Photomultiplier Tube and by finding the delay between the time the Led signal was emitted and the Photomultiplier Tube picked the signal up gave a time of flight for light over a given distance, from there it was a simple rate * time = distance calculation.


The procedure and analysis of this laboratory are contained in my lab notebook here [[1]].

The final value I calculated for the speed of light was 2.88e8 meters/second +/- 1.31e7. This compares to the accepted value for the speed of light of 2.9979e8 m/s with a percentage error of 3.9%. When time walk was not accounted for I ended up with a value for the speed of light which was a positively pedestrian 1.605e8 m/s.

SJK 15:39, 19 December 2009 (EST)
15:39, 19 December 2009 (EST)
Looking at cell I-72 of your spreadsheet, it looks like you are using standard deviation instead of standard error of the mean here. Since there are four measurements, this puts you off by a factor of 2 in overestimating your uncertainty.


The value for the speed of light I calculated for this experiment was quite close to the accepted value for this quantity. There are several ways that this experiment could have been improved.

  • First, i would separate the polarizer from the PMT so that when the polarizer is realigned there is no chance that the distance between the LED and the PMT would not have the opportunity to change.

  • Second, I would motorize the movement of the LED to increase both the precision and accuracy of the movement and reading of position to get the best data possible.

  • Last, The way the voltage is taken off the oscilloscope screen is not exact. The PMT seems to pick up quite a bit of noise so the voltage reading was quite susceptible to wandering. It seemed that the measure function on the scope was useless, as it stated a value that would change by .1 volts without anything happening to the system.

Working through these challenges and getting a quality value for the speed of light was quite encouraging. Overall the experiment was informative and enjoyable.

SJK 15:36, 19 December 2009 (EST)
15:36, 19 December 2009 (EST)
Glad you liked it and looks like you got good data! A comparison of whether accepted value is in or close to your range of uncertainty is again missing.