User:Manuel Franco Jr./Notebook/Physics Lab 307/2008/09/17

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Measuring the Speed of Light

Lab Partner: David


The objective is in the title. We are going to measures the speed of light using such materials, and follow a certain procedure as described below.


SJK 02:47, 6 October 2008 (EDT)
02:47, 6 October 2008 (EDT)
This section is outstanding!

I notice now, though, that you based it on Jesse's speed of light lab. You cite his lab writeup at the bottom of the page, which is very good scientific practice. However, with just the citation at the bottom, it's not obvious that you modeled this section after, you should give him credit by saying something like "Equipment (based on Jesse's notebook")
  • NIM (Nuclear Instrument Modules) Bin: Model NQ-75. It contained:
  1. TAC (Time Amplitude Converter): Model 567 from EG&G Ortec
  2. Delay Module : Model 2058 from Canberra
  3. DC Power Supply that ranges from 0-5000V and 0-5mA: Model 315 from Bertan Associtates Inc. {1}
  • DC power supply that ranges from 0-200V and 0-0.2A: Model 6207A from Harrison Lab {2}
  • DSO (Digital Storage Oscilliscope): Tektronics TDS 1002
  • An LED (Light Emitting Diode),Polarized
  • PMT (Photomultiplier Tube): All it had marked was N-134, Polarized.
  • A Tube about 3 to 4 meters long made out of cardboard
  • 2 - 2m meter sticks taped together
  • Several BNC wires
SJK 22:24, 6 October 2008 (EDT)
22:24, 6 October 2008 (EDT)
Excellent use of photos

Here are some photos of the equipment used:


When we arrived everything was already setup by the Lab prior to us, but we still had to review the equipment and wires to check if they were in the correct place. We were aided by Koch, but also by the Lab Manual, Section 10.

At this point everything is turned off, and all the dials are set to there lowest settings. The TAC, the delay, and the power supply {1} were in the NIM bin. The second power supply {2} was connected to LED. The LED was taped to a meter stick which was taped to another meter stick. It was inserted into the cardboard tube on one end. The LED was contected to the TAC's "start."

As for the other end, the PMT was inserted there. The Lab manual warns, and I quote, "Warning! This experiment uses a photomultiplier tube (PMT) that will be ruined if exposed to ambient light when at operating voltages. Please be careful - the PMT must never be exposed to any bright light source while its high voltage is turned on. Permanent damage will result!" Lab Manual, Section 10.2. The PMT has three connections, and we connected the following. The PS{1} was connected to the PMT. Then, the PMTs second connection was connected to the delay. From the delay it was connected to the "stop" input of the TAC, and then from the TAC output into the DSO in channel two. As for the third connection it was connected to channel one in the DSO.

So, everything was correctly connected, now we were able to turn on all the instruments, and begin gathering data.


After checking if everything was set up correctly, we turned on the TAC, both power supplies, and the DSO. So as instructed by the Lab manual we set the PS{1} to about 1800 V to 2000 V. We decide set PS{1} at 1990 V, and PS{2} to 150 V to see if we were getting any data at all. Nothing... With help from Koch, we figured out that the PS{2} was set low, and that our DSO was out of range. So we set PS{2} to 170 V (+/- 1 V), and ranged the DSO. Finally, we obtained the two graphs necessary to record data, as shown in Photo 2.

SJK 02:50, 6 October 2008 (EDT)
02:50, 6 October 2008 (EDT)
OK, I trust you understood time walk...but I don't understand what you're saying in this paragraph
Time walk is a new time that I became familiar with in this experiment. The speed of light travels approximately a foot per nanosecond; so the instant we turned on the equipment and the LED emitted that pulse, light had to travel several feet to light up the LED. That delay in light would ruin our measurements. The delay would help us make a reference to how light was moving. As for the TAC, it had to be trigger at a fixed voltage due to the polarization in the PMT and LED. The lab manual goes more in to detail over this process.

As we moved the LED and PMT we change the distance between them, and we measured the voltage in which we get the time from that. Whether if we bring the LED closer to the PMT, or if we pull the LED away from the PMT the speed of light is constant. With this data we can make distance and time graph. With that we can obtain a slope or velocity, hence our measurement of the speed of light.


PS{1}: 1908 V (-/+ 1 V) on all measurements

PS{2}: 170 V (-/+ 1 V) on all measurements

1st Measurement

PMT Reference Voltage: -648mV (-/+ 1 V)?

Range (TAC): 50 ns

Time delay: 0 nsecs

1.) 150 cm   2.56 V
2.) 140 cm   3.48 V
3.) 120 cm   3.68 V
4.) 100 cm   3.36 V
5.) 80 cm    3.12 V
6.) 60 cm    2.69 V 
7.) 50 cm    2.12 V

2nd Measurment

PMT Reference Voltage: -1.12V (-/+ 1 V)

Range (TAC): 50 ns

Time delay: 0 nsecs

1.) 60 cm  2.10 V (+/- 0.04 V)
2.) 50 cm  2.04 V (+/- 0.04 V)
3.) 40 cm  2.02 V (+/- 0.04 V)
4.) 30 cm  Same as above 
5.) 20 cm  1.95 V (+/- 0.04 V)
6.) 10 cm  1.84 V (+/- 0.04 V)

3rd Measurment

PS{1}: 1908 V (-/+ 1 V) PMT Reference Voltage: 850 mV(-/+ 1 V)?

Range(TAC): 50 ns

Time delay: 0 nsecs

1.) 160 cm  3.00 V (-/+ 0.04 V)
2.) 150 cm  2.94 V (-/+ 0.04 V)
3.) 140 cm  2.90 V (-/+ 0.04 V)
4.) 130 cm  2.87 V (-/+ 0.04 V)
5.) 120 cm  2.82 V (-/+ 0.04 V)
6.) 110 cm  2.78 V (-/+ 0.04 V)
7.) 100 cm  2.70 V (-/+ 0.04 V)
8.) 90 cm   2.66 V (-/+ 0.04 V)
9.) 80 cm   2.54 V (-/+ 0.04 V)
10.) 70 cm  2.52 V (-/+ 0.04 V)
11.) 60 cm  2.46 V (-/+ 0.04 V)
12.) 50 cm  2.38 V (-/+ 0.04 V)

4th Measurment

Measurments with time delay

PS{1}: 1900 V (+/- 1 V)

PMT Reference Voltage: 1.2 V

Range (TAC): 50 ns

Time Delay: 2 nsecs

1)130 cm   2.38 V (-/+ 0.04 V)
2)120 cm   2.34 V (-/+ 0.04 V)
3)110 cm   2.22 V (-/+ 0.04 V)
4)100 cm   2.14 V (-/+ 0.04 V)
5)80 cm    2.04 V (-/+ 0.04 V)

Data Analysis

Measurements 1,2, and 4 were ignored due to trial and error. Measurement 3 was most accurate out of all data accumulated. All data is in this spread sheet. I followed Jessy's format to plot and analyze my data. My Slope, or measurement for the speed of light is 352261684.3 m/s.

SJK 23:11, 6 October 2008 (EDT)
23:11, 6 October 2008 (EDT)
I looked at your excel sheet and a bunch of the cells show "0" for values...?


Lab Summary

This is my lab summary.


  1. Lab Manual, Section 10.1-10.4
  2. Jessy's Lab Notebook
  3. Knockel's Lab Notebook
  4. Gooden's Lab Report