# Physics307L:People/Klimov/Lightspeed

SJK 17:00, 17 December 2008 (EST)
17:00, 17 December 2008 (EST)
Excellent job on this lab. Very nice data and analysis and identification of the need for a delay.

# Speed of Light

The speed of light is a fundamental constant in nature, and it appears in most branches of physics. The constant is the framework of special relativity, governing the dynamics of relativistic objects and the fabric of space-time itself. In addition, the constant arises naturally in Maxwells equations, often disguised as the inverse of the square root of the product of the permittivity and permeability of free space. Clearly, measurement of the constant is an important goal in physics, from both theoretical and experimental perspectives.

In this lab, we set out to measure the speed of light by measuring the time delay between the emission and reception of short light pulses. The pulses were be generated by a light emitting diode (LED) and received by a photomultiplier tube (PMT). This time delay was measured as the relative distance between the PMT and LED was changed. Three sets of data were obtained, using three slightly different procedures. During the first trial, we moved the LED outwards from the PMT. During the second trial, we moved the LED towards the PMT. And finally, during the third trial, we moved the LED outwards from the PMT, this time delaying the PMT signal by 10 ns (the reason for doing so is described in detail in the 'Discussion Section' of my lab notebook).

## Results

$c_{act}=2.99 \cdot 10^{8} m/s$

$c_{1}^{0ns Delay}= 2.54(13) \cdot 10^{8} m/s$

$c_{2}^{0ns Delay}= 2.56(13) \cdot 10^{8} m/s$

$c_{3}^{10ns Delay}= 2.99(11) \cdot 10^{8} m/s$

## Conclusions

• Although I do not believe we are ready to submit a single value for the speed of light, I am happy with our progress. I believe that we carried out our experimental procedure well, obtaining high quality data which produced great linear fits. As we can see, in our first two trials, the actual value of the speed of light lies about 3.5 SEMs away and in our last trial the actual value of the speed of light is contained comfortably within the SEM.
• Clearly, however, we have discrepancy between our first two trials and our final trial, which is the result of a 10ns time delay imposed on the PMT signal. While I suspect that such a time delay would improve our results (see Discussion), I don't believe we can firmly conclude that that value is our best guess of the constant. As I mention in my report, I believe in a real experimental situation this would be a good time to take some more measurements -- and definitely not a good time to report final values of any sort. However, with that said I don't think it would take more than a few more hours in the lab to come up with even better data, if we were inclined to do so.