Physics307L F09:People/Martin/Notebook/071010

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Measuring the speed of light


To measure the speed of light (see title)


There is a better, more scientific description of how this expirment works on the previous courses website lab manuel for speed of light. In case you find that explanation too hard to understand here is the speed of light apparatus operation for dummies. For this expirment a LED is needed (obviously light is needed if we are to measure the speed of it). A power supply is connected to a the LED and the LED is then connected to time-to-amplitude converter (TAC). A photomultiplier tube is set up opposite the LED and is connected to the TAC through a time delay appratus. The delay appratus is used to make sure that the light sent signal is recieved before the light recieved signal. The TAC device is conneced to a oscilliscope which yields a voltage reading. This voltage reading is related to the time that is takes for the light to be sent then recieved. Use the differences in position over the differences in time (remember to convert the voltage to time correctly, ask if necessary) and get the result for the speed of light.

Time Walk

Time walk is difficult to explain but last years lab notebook does a pretty good job at it. to make sure that we were avoiding this major source of error we made sure that the amplitudes of the light we recieved were the same (the reading on channel one of the oscilliscope).


  1. TAC Model 567 manufactured by EG&G Ortec
  2. Delay 2058 manufactured by Canberra
  3. Power supply Model 315 DC power supply 0-5000V, 0-5mA Manufactured by Bertan Associates Inc.
  4. Oscilliscope Tektronics TDS 1002
  5. LED Power Supply: Model 6207a mfd. by Harrison Industries, 0-160V, 0-0.2A
  6. Photo mult tube: N-134


copied from jesse's lab notebook SJK 01:20, 25 October 2007 (CDT)
01:20, 25 October 2007 (CDT)
Overall you guys took some great data! Looks like your careful work paid off in some good measurements.

since we are using a meter stick and measuring centimeters assume that the error in measurement is +/- 0.001 meter for all of the length measurements.

2 nSec delay
Baseline measurement:  -1.30 V
            (Channel 1 Min Voltage at stick measurement of 80 cm with average of 128 measurements)
80 cm Channel 2 Max: 3.56 V +/- 0.02 V
100 cm Channel 2 Max (channel 1 min reads between -1.30V and -1.28V): 3.44 V +/- 0.02 V
120 cm Channel 2 Max (channel 1 min reads between -1.30V and -1.28V): 3.28 V +/- 0.02 V

0 nSec delay
Baseline measurement: -800mV
120 cm: CH2Max: 4.34V +/- 0.02V
110 cm: CH2Max: 4.40 +/- 0.04V  (CH1Min: 800mV)
100 cm: CH2Max: 4.50 +/- 0.02V  (CH1Min: 800mV)
NOTE: CH1 Resolution was very bad.  This data should be disregarded.

0 nSec delay
PMT Reference: CH1Min: -800mV +/- 8mV
70 cm: CH2Max: 4.74V +/- 0.02V
80 cm: CH2Max: 4.70V +/- 0.02V
90 cm: CH2Max: 4.60V +/- 0.02V
100 cm: CH2Max: 4.54V +/- 0.02V
110 cm: CH2Max: 4.50V +/- 0.02V
120 cm: CH2Max: 4.42V +/- 0.02V
130 cm: CH2Max: 4.37V +/- 0.03V

Day two

0 nSec delay
PMT Reference: CH1Min: -800mV +/- 8mV
140 cm: CH2Max: 4.40V +/- 0.02V
60 cm: CH2Max: 4.82V +/- 0.02V
50 cm: CH2Max: 4.95V +/- 0.03V
40 cm: CH2Max: 4.96V +/- 0.03V



SJK 01:19, 25 October 2007 (CDT)
01:19, 25 October 2007 (CDT)
How did you obtain the uncertainty on the slope? While I personally know you used excel, since you didn't link to your excel file, or explain your methods, it wouldn't be clear to other readers (or even yourself a few weeks from now) how you did that!
  • slope: 306301260.3 m/s +/- 1.83E7 m/s
  • wikipedia defination of the speed of light: 299792458 m/s
  • percent error: 2%