User:Emran M. Qassem/Speed of Light

From OpenWetWare
Jump to: navigation, search



In this lab we measured the speed of light. Using a Time of Flight method and a time to amplitude converter, we were able to use an oscilloscope to take a few measurements of time and distance and find the ratio of the change to find a slope which represents 1/c or the inverse of the speed of light, using least squares.


After understanding the setup and method of data collection, we moved the LED along inside the tube and recorded the voltage values we gathered from the oscilloscope for each change in distance into a spreadsheet.

We ran 2 trials, the first with 16 measurements of 10cm each, and the second with 8 measurements using 20cm each.

Using the TAC conversion rate of 1v = 10ns, recorded our measurements for each point.

Using this data, we were able to use the LINEST function (least squares) to find the slope, which we inverted to find our observed speed of light.


We gathered data from two trials, having a result of 31.5 (12) cm/ns and 32.1 cm/ns +1.9 cm/ns -2.1 cm/ns. These results were a little more than 1 sigma away.


With an error of a little more than 1 sigma, and both trials being above the accepted value, we can safely say this is caused by systematic error. This experiment is known to suffer from the systematic error called time walk. The TAC triggers on the start and stop signals. While the start signal will remain the same, the stop signal, which is received from the PMT, will change in amplitude as the LED gets closer or further away. This change in amplitude will cause a trigger to occur sooner or later than the previous time, unless the amplitude is kept the same.


We learned about time walk, how to look up information in device white papers/manuals, and how to use an oscilloscope even better than before. Our results were as expected, as this experiment is known to have systematic error.