Physics307L F09:People/Long/Lightspeed

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Speed of Light Lab Summary

SJK 23:53, 4 October 2009 (EDT)

23:53, 4 October 2009 (EDT)
Excellent job on this lab and good summary. See comments below for how to improve summary next time. Also, see Tom's primary lab notebook (and his summary) for comments that apply to both of you. Good work!

In the speed of light experiment, my partner Tom and I calculated a value for the speed of light using an LED circuit and a Photomultiplier tube (PMT). Our procedure for this lab was based on instructions provided by Professor Gold’s manual. My lab notebook including raw data, and our setup can be found here.

Data Summary

The following data was obtained by measuring the time delay between the LED emitting a pulse, and the PMT receiving the signal. The delay is then converted to a voltage by a time amplitude converter, which we then measured through an oscilloscope. We repeated this procedure with varying distances, a total of six trials. See my lab notebook for more details on setup, and raw data.

SJK 22:36, 4 October 2009 (EDT)

22:36, 4 October 2009 (EDT)
I know you reported this before our class last week, but some comments: (1) Excellent job reporting results with statistical uncertainty and units (2) Less significant figures would be better. E.g. (2.95 ± 0.04). (3) Good job reporting the accepted value...but where do you get that value? You need to cite that so the reader can understand what you're reporting. Also, you can clarify if that's the value in vacuum, air, cardboard tubes, etc...or whether it doesn't matter. Finally, in future labs, not only will you report your results this way, but you'll discuss whether your results are statistically consistent with the accepted value (using gaussian probabilities of normally distributed mean). I really like how you explicitly state what your error bars are derived from--that is great! Otherwise, reader would have assumed they indicated 1 standard deviation...good job!
  • Our measured mean speed of light based on the six trials:
[math]2.9461 \pm 0.0437\times 10^{8} m/s[/math]
  • The accepted value of the speed of light is:
[math]2.9979\times 10^{8} m/s[/math]

The uncertainty in our voltage measurements is the variation of the signal on the oscilloscope from our reported value. It is not the standard 68% confidence uncertainty.


Initially, the setup and procedure was very difficult to figure out, but after reading over instructions and manuals, we got a good routine down for recording the data. It was a good experiment because we got a chance to learn how to use a variety of new equipment. One of the most difficult concepts of the lab for me, was "time walk", this also made for a major cause of potential error in our measurements. Time walk is an effect caused by moving the LED closer and closer to the PMT, as we moved the LED closer to the PMT, the intensity of the light received changes. As the intensity changes, the signal on the oscilloscope also changes. Since we are measuring the amplitude of the pulses on the oscilloscope, we need to make sure that the signal remains in the same spot on the oscilloscope for slope triggering to take place. We compensated for the time walk effect by rotating the polarizer on the front of the PMT, as we moved the LED closer, the rotation of the polarizer reduced the intensity, to keep the signal amplitude maximized.SJK Steve Koch 22:52, 4 October 2009 (EDT)

Steve Koch 22:52, 4 October 2009 (EDT)
Note to both of you: You're right, this IS a very difficult experiment to get to the point of taking data with very little systematic error from time walk! It looks like you did a great job, based on your final values. Especially impressive is how much you figured out on your own because I missed so much of lab time. Sorry about that -- I would have enjoyed working with you more. But kudos to you two for figuring it out and doing an excellent job!

I was very pleased with our results, although I don't quite fully understand uncertainty and error calculation, our measurement seemed very close to the accepted valueSJK 23:52, 4 October 2009 (EDT)

23:52, 4 October 2009 (EDT)
Good for you for realizing that you still have things to learn about error analysis! And given how hard you're working, I know that you will learn those things and gain a deeper understanding

. Originally I had a hard time understanding time walk, and we looked around on the web, and surprisingly could not find much info at all. For all but the last trial, my role in the lab procedures was to rotate the PMT, and take readings on the oscilloscope. This task would've been much easier if the PMT was fixed in the end of the tube with a separate polarizer to rotate. Tom and I used the google docs for our data table again, and I was stoked with how well it worked out.


Ryan's Lab Notebook

Professor Gold's Manual

Ryan's OWW Page

Tom's OWW Page

Physics 307L