# User:David J Weiss/Notebook/people/weiss/Lightspeed

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# Summary of the Speed of Light Lab

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• The main goal of this lab was to measure the speed of light. To do this we set up a photomultiplier tube that receives and measures a light pulse generated by an led. We do this while moving the led closer to the photomultiplier closer to the led and recording the different voltages corresponding to the distance the light travels. One of the main difficulties in this lab was dealing with the so called time walk. This is when you have to trigger off of different amplitudes in order to correct the voltages you are measuring due to the fact that more light reaches the photomultiplier when you move the led closer to the photomultiplier. To adjust for this you need to choose an amplitude and then when you move the led you need to adjust the photomultiplier by rotating it in respect to a filter so that the amplitude matches the one that you were starting with. After you figure this out the lab becomes significantly easier.
• In this lab I calculated the speed o'light using 2 different methods. One of the methods was moving the led in 10 cm increments and going to a distance of 100 cm. In the other method I moved the led in increments of 25 cm and went to a distance of 150 cm. The values I obtained are different, but not too much. The value I obtained using the first method was $3.285(.025)\times 10^8\frac{meters}{sec}$ witch had and error of about 9.58% witch is not too bad. This was probably due to the fact that there was significant noise in the oscilloscope when we were taking the data and we used the averaging function to try to reduce the amount of noise in the data but then that could put some more error in the data but what can you do. In the second method I obtainment the value of $2.8767(.0127)\times 10^8\frac{meters}{sec}$ which had a error of about 4.04%. I cant tell witch method is better since the error on each is less than 10% on each and if you were to do the experiments over and over again i would suspect that the error on each would probably reduce and equal out on the two.