# Physics307L:People/Mahony/Lightspeed

## Speed of Light Lab Summary

SJK 23:55, 4 October 2009 (EDT)
23:55, 4 October 2009 (EDT)
Excellent job on this lab! Good summary, see comments below on how to improve it next time. Also, see comments on your primary lab notebook, and take a look at Ryan's summary for further comments.

In this lab, my partner Ryan and I measured the speed of light. We did so by measuring the delay between a triggered pulse from an LED and the corresponding received pulse generated by a photomultiplier tube (PMT). The procedure we followed is outlined in Professor Gold's manual.

## Results

SJK 22:38, 4 October 2009 (EDT)
22:38, 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). Note how Ryan described what your error bars mean right in his summary -- since they're not the "typical" standard error of the mean, that is important to point out in the summary (even though I'm sure I'll find that you describe it well in your primary notebook).

Our measured speed of light was:

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

The accepted value is:

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

For our raw data and to see how we arrived at this value, see the lab notebook.

## Conclusions

I was very pleased with this lab. Despite having to compensate for the effect called time-walk, which is a problem encountered when trying to trigger off of signals of differing amplitudes, we managed a reasonable measurement for the speed of light. In fact, our measured value was accurate to the two most significant digits.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!
In this lab I became familiar with lab equipment such as Time-Amplitude Converters (TACs) and photomultiplier tubes (PMTs). I started to learn how to analyze data at a basic level. I also learned the importance of scrutinizing your data and the procedure you took to determine if there are any major sources or error or if your data is relevant. I still have a bit to learn about data analysis, including more advanced techniques and the significance of the standard error of the mean.SJK 22:43, 4 October 2009 (EDT)
22:43, 4 October 2009 (EDT)
I am really glad you have your sights set on learning more and more about data analysis! Off to a great start, and I can tell that you're going to learn a lot this semester. Plenty of opportunities to try new things with your Millikan experiment!
The lab setup was intuitive once we understand what was going on, which actually ended up taking quite some time. Ryan and I thought of several several minor improvements, though I would like to highlight one. We though it would be a good idea to detach the polarizer from the PMT so it can be rotated separately. This change could possibly reduce error.SJK 22:44, 4 October 2009 (EDT)
22:44, 4 October 2009 (EDT)
I agree...ideas, though, on how to uncouple them without letting light into the tube?