Physics307L:People/sosa/Light of speed

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Hyperspace.jpg Speed of Light Lab <html><img src="/images/9/94/Report.png" border="0" /></html> Notebook
SJK 23:24, 6 October 2008 (EDT)
23:24, 6 October 2008 (EDT)
Great job with your primary lab notebook and data taking! The summary page is not in the right area, please ask me about this. Also, please see Manuel's notebook for more comments from me.


The objective of this experiment is to determine the speed of light with the greatest possible accuracy.

Lab Partner: Manuel Franco


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 ( A manual was not available in the lab, so this manual may be helpful to future experimentalists)
  2. Delay Module: nSec Delay model 2058 mfd. by Canberra
  3. DC Power Supply that ranges from 0-5000V and 0-5mA: Model 315 from Bertan Associtates Inc.
  • DC power supply that ranges from 0-200V and 0-0.2A: Model 6207A from Harrison Lab
  • DSO (Digital Storage Oscilliscope): Tektronics TDS 1002
  • An LED (Light Emitting Diode).
  • PMT (Photomultiplier Tube): All it had marked was N-134. See Wikipedia's entry on PMT for more information.
  • 2 polarizing filters.
  • A Tube about 3 to 4 meters long made out of cardboard
  • 2 - 2m meter sticks taped together
  • Several BNC wires

Some images of the materials used

Setup and Connections

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 Dr. Koch and the TA Aram, but also by Dr. Caves' Lab Manual, Section 10.

  • The DC Power Supply (Photo 5) supplies power to the LED. This DC power supply is connected to the first input on the TAC. The LED is inside the cardboard tube and attached to the meter stick so we can vary its distance. The LED also also has a polarizing filter in front.

  • The PMT is located at the other end of the cardboard tube. It is connected to its own power supply located in the TAC. The PMT is also connected to Channel #2 in the oscilloscope. Another polarizing filter is nearby the PMT.

  • TAC: The delay module's output is connected to the second input on the TAC. The TAC has its output connected to the Channel #1 of the oscilloscope.


After checking everything was set up correctly, we turned on the TAC, both DC power supplies, and the oscilloscope. So as instructed by the lab manual, we set the TAC power supply to about 1800 V to 2000 V like the manual suggests. We decide set the TAC power supply at 1990 V, and the other DC power supply to 150 V. We didn' get any graph on the oscilloscope. After trying to determine the reason for this we decided to ask for help. With help from Koch, we figured out that the DC power supply connectted to the LED was set too low, and that our DSO was out of range. So we set the DC power to 170 V (+/- 1 V), and ranged the DSO. Finally, we obtained the two graphs necessary to record data, as shown in Photo 2.

Time walk was a new concept we had to learn for 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 provide the wrong data. 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.

Data Taken

DC Power supply: 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 (BEST DATA)

  • 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 Measurement

Some Measurements with time delay

  • 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)

Results and 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 our data is in this spread sheet. We followed Jessy's format to plot and analyze the data.The 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...?

graph of Rydberg constant for various values of n for each spectrum line


SJK 22:48, 6 October 2008 (EDT)
22:48, 6 October 2008 (EDT)
You need to report your estimated uncertainty on your final measurement...this would allow you to say whether the final value is consistent with the accepted value
Our result was ~17% deviated from the accepted value. We might have obtained better results if we had taken more measurements to compare. Unfortunately due to lack of time we weren't able to. Overall I think it is a hard laboratory. Many new concepts are introduced, but I also think it is really important. How many people can say they have measured the speed of light? One of the reasons I want to become a scientist is to check things myself, and not be told that X or Y is true just because someone said it a long time ago.SJK 22:46, 6 October 2008 (EDT)
22:46, 6 October 2008 (EDT)
I'm glad you point out that this experiment is fun and the excitement of being to able to measure such an important physical constant!


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