IntroductionSJK 01:17, 1 November 2008 (EDT)
Our set up is such that we measure the time difference between two different points of light emmission. The light is emmitted from an LED that we can push in and out of a tube. At the other end of this tube is a photomultiplier that converts this pulse of light from the LED into a signal that is imputed into a Time-Amplitude-Converter (TAC). The TAC converts the time difference between the start and stop signals (the LED pulse is start, the light the PMT received is stop) and outputs this time difference as a voltage. We read the voltage with an oscilloscope with the understanding that the TAC is outputing a ratio of 10 V per 50 nS.
With this in mind we can plot position vs. time and find the slope of the best fit line.
Please refer to my lab notebook for in depth discussion of the set up, the time walk, the data taking method, and such.
Accepted Value of the Speed of light in a vacuum:
Of importance to the following results is a discussion on the method of each trial. Each of the four trials below was characeterized by different changes in position of the LED. In the fourth trial we did not use the time walk correction of the polarizers. This of course validated the importance of the time walk correction, which takes into account the changes of light intensity as the LED comes closer to our PMT. Please refer to the lab notebook for more detail.
Trial 1: Changes in distance increased rapidly from the initial point.
- cup = (4.2302) 108 m/s
- cdown = (1.5972)) 108 m/s
Trial 2: Small changes in distance.
- cup = (1.7924) 108 m/s
- cdown = (1.0481)) 108 m/s
Trial 3: Larger changes in distance, but not as large as trial 1.
- cup = (4.9498) 108 m/s
- cdown = (2.0002)) 108 m/s
Trial 4: No time walk correction.
- cup = (6.1817) 108 m/s
- cdown = (2.2201)) 108 m/s
What I notice with my data is the importance of large changes in distance of the LED, and the necessity of the time walk correction. Trial 1 is obviously the closest value, and this trial was characterized by changes of distance that were increasingly large.