Physics307L F08:People/Martinez/Speed of Light

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Speed of Light

  • For this lag we conducted an investigation for the most fundamental constant: the speed of light. Our method followed the lab 10 from the lab manual. We used a direct time of flight measurement of a pulse of light from an LED over the span of our yard sticks in order to estimate this quantity. The signals that we recorded were from the direct signal and the PMT (photomultiplier) signal, an electrical signal created from our light pulses after being detected by our PMT and transduced to electricity by the photoelectric effect which was put through the TAC (time to amplitude converter) which gives us a voltage that corresponds to the time from start stop time of the light pulse. The PMT emits electricity based on the intensity of the light it receives and therefore we were required to rotate a polarizer to limit the amount of light emitted and keep that from affecting our ToF measurement. The information that we received (cm/V) was plotted for a best fit slope, and multiplied by calibration data (V/ns) to receive our value for the quantity of the speed of light- this information is on our excel page. Our biggest problem with this lab was that we spent the entire first class period measuring data from noise found at a higher than needed resolution - by after figuring out what we were doing we conducted some measurements allowing for the time walk as a comparison against the data we actually intended to use, but our initial data was collected with too little resolution for our amplitude and our data looked ridiculous, with very little time left we corrected for this error and took one set of decent data.
  • wiki page for our notebook data. User:Michael R Phillips/Notebook/Physics 307L/2008/10/01
  • Our data gave us the answer for our quantity by best fit slope from linest (and error from index) as: 23.234(4)cm/ns)SJK 03:20, 22 October 2008 (EDT)
    03:20, 22 October 2008 (EDT)
    Does your intuition tell you that you were really that accurate (.01%-ish)??? I think you have an error in your error formula...from your data I get something like 1 cm / ns uncertainty in the slope.

    I do like that you're using "cm / ns" instead of "m/s" ... seems much more practical, especially for this lab.
    the accepted value of the speed of light is 29.9792 (cm/ns)so our data was well off from the accepted our sources of error could have been: Not measuring the centimeter value of our yardsticks with enough accuracy, although this would have been a minor infraction; not rotating enough the polarizer: which was possible considering that for our good data we calibrated this data by eye. Most of our problems seemed to be caused by our misinterpretation of the scale for which we would record the amplitude with enough accuracy.
  • I learned a lot about mostly the photomultiplier tube - I was really interested by its operation, I am still however very unclear about the circuitry involved in our time to amplitude converter, and would like to understand its operation better. Overall this lab was frustrating and confusing so there was not enough time to find anything particularly interesting. I think I also learned to ask for help more often, because winging it got us into a lot of trouble with data mistakes that could have been more easily avoided if we had gotten help.
  • My suggestion for next year is to add more information about what the output data looks like on the oscilloscope - the manual was really lacking in information about data gathering and interpretation, and really truncated in general.