Physics307L F09:People/Long/Lightspeed

Speed of Light Lab Summary
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.


 * Our measured mean speed of light based on the six trials:


 * $$2.9461 \pm 0.0437\times 10^{8} m/s$$


 * The accepted value of the speed of light is:


 * $$2.9979\times 10^{8} m/s$$

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.

Conclusion
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.

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 value. 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.

Links
Ryan's Lab Notebook

| Professor Gold's Manual

Ryan's OWW Page

Tom's OWW Page

Physics 307L