User:Tyler Wynkoop/Tyler's Page/SOL

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Speed of Light Lab Summary

Goals and Procedure

In this lab, the goal was to measure the speed of light. This must be done by non-conventional means. In order to measure the speed of light an LED is shown on a photomultiplier tube (PMT) at a known distance. The difference in time from when the LED emits a pulse and the PMT receives the pulse is recorded. Then the distance between the two is divided by this time. The trouble here is that the electrical signals traveling to the oscilloscope (which receives the data) are slower than the light pulses. A time delay must then be put in the path of the shorter cable, in this case the cable from the PMT to the oscilloscope, in order to match the time. In the experiment, various distances were used for the best accuracy. The procedure with this lab is exceedingly simple. The distance between the PMT and the LED is adjusted, and the time is recorded. The intensity of the light can cause issues with the time measurement, so polarizers are put in the light path to control the intensity.


I measured the time by using a time-to-amplitude converter (TAC). When the TAC receives a signals from the PMT or the LED, it compares them, and converts the difference into a voltage. In my experiment, one volt corresponds to 1 nanosecond. The average value we found for C was .86x10^8 ± .26x10^8, and an error of 71.3±8.7% I would like to note, however, that the longer the distance between the PMT and the LED, the more accurate our measurement for 'c' became. This error is very large in comparison to previous labs, but the order of magnitude was correct. Considering something this difficult to measure, our data was within acceptable values.


Systematic error was huge in this lab. Firstly, the voltages yielded by the TAC were extremely unstable. Often varying ±.75 of a volt. This means that the times used in my calculations, and therefore, the final data, could have had an margin of error of several hundred percent. Another source of error, was that the oscilloscope has a limited sensitivity. All of my results were exactly .04 volts away from the previous measurement. Finally, the PMT could be easily influenced by outside light sources, skewing data.