# Physics307L F09:People/Ierides/Speed Of Light

## Speed Of Light

SJK 00:42, 16 November 2009 (EST)
00:42, 16 November 2009 (EST)
Overall, excellent job on this lab!

The main overview and summary of the procedure and lab is provided by this link:

Prof. Gold's Lab Manual
The following link leads to my Balmer Series Lab entry that includes the data, procedure, and observations that were made:
Speed of Light Lab Notebook Entry

Please note that Alexandra Andrego was my lab partner for this lab. Her version of this lab can be found here. You can also find her lab summary by following this link.

### Data Summary & Observations

SJK 00:39, 16 November 2009 (EST)
00:39, 16 November 2009 (EST)
You have too many digits of precision on your final value and uncertainty. It would be better written as (3.00 ± 0.05) E8 m/s or 3.00(5) E8 m/s. The extra digits don't have much meaning and are distracting.

The purpose of this lab was to measure the speed of light using a high speed detector and photomultiplier tube (PMT) whilst moving the detector to varying distances between 0 cm and 100 cm from the PMT and then manually adjusting the PMT to align the polarizers as to keep consistent data. After completing several trials of this process, we came to the measured average value found in the Calculations section of my lab notebook [1]:

$c_{measured}\simeq(3.0014\pm0.051)\times10^8 m/s\,\!$

where the $\pm0.051\times10^8 m/s\,\!$ includes our range of measured values calculated using our uncertainty in the original slope. With percent error from the accepted value $c_{accepted}=299,792,458 m/s \,\!$ of:

$\% error\simeq0.116%\,\!$

All uncertainty in our values is described in the Notes About Uncertainty section of my lab notebook (link above).

SJK 00:41, 16 November 2009 (EST)
00:41, 16 November 2009 (EST)
I am pretty sure you are missing any statement about whether your measurements are consistent with the accepted value for the speed of light. That's important! Your measurements are consistent, because the accepted value is well within your range. In fact, you could go even further and say that your very small percent error is evidently very lucky, given that your uncertainty is much bigger (1.7% uncertainty, compared with 0.1% relative error).

### Conclusion

In conclusion, I believe this lab went quite well, with a relatively small percent error from the accepted value of the speed of light. Although there were many ways in which systematic error could have existed, my partner and I have managed to overcome them in such a way as to achieve a measured value very close to the accepted value. I would probably recommend that the meter stick measurement would be placed directly on a rounded value instead of the 0.8 cm offset to reduce error. Also, having to adjust the PMT by hand in such a way as to align the polarizers and maximize a semi-steady amplitude square wave on the oscilloscope was a source of error as well. I'm not sure as to how to fix that personally but if it could be fixed as well, that would reduce error as well. Another source of error that could be reduced slightly is if there was a zoom function on the oscilloscope for a better view of the graph function. Overall, I believe this lab went quite well. Although the speed of light is a universally fundamental constant, it can be easily measured with the right equipment and procedure.