Speed of Light

Light from a modulated He-Ne laser is detected at the beginning and end of a measured path. The AC signals from photodetectors at these two points are compared on an oscilloscope to measure the phase difference and from that the velocity of the light.

A beam splitter is used to divide the beam for detection at begining and end of the measured path. Plane, front-surfaced mirrors are used to deflect the beam to get as long a path as possible.

  1. Equipment needed.
  2. Basic setup.
  3. Experimental problems
  4. Troubleshooting
  5. Sample data
Example setup
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Equipment Needed

Modulated Helium-Neon laser, Metrologic Model ML-868
Double photo-detector and amplifier for detecting laser at beginning and end of path.
Beam splitter
Signal generator for modulating the laser.
Oscilloscope
Front-faced plane mirrors and mirror holders
Converging condenser lens for the second photodetector.
9V battery for detector box.
Frequency counter
Several BNC cables and about 2 ea red and black banana leads abt 18"
Four BNC tees
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Basic Setup

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Experimental Problems: Speed of Light

1. Frequency must be high to give a measurable separation. 1.4 MHz was used by W95 group. A triangle wave made reading the wave position easier.

2. Mirror shake was a problem. Hopefully the use of optical tables and mounts will help with that.

3. The signal from one photocell would bleed into the other, making the signal indistinct. The W95 group used the signal generator as a standard and measured the two photocells against that standard and took the difference.

4. The scope is difficult to read because of vibrations and a fuzzy signal.

5. Internal delays of the triggering of the oscilloscope? caused the line of best fit of the data to not cross zero. To overcome this problem, the slope of the data at different distances with respect to time was taken to get the speed of light.

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Trouble- shooting
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Troubleshooting: Speed of Light

There are a number of experimental problems inherent in the speed of light experiment. Here are some further comments about problems encountered.

1. The high gain amplifier used to amplify the signal from the photodetectors tends to oscillate.

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Sample Data

Modulation: 600 kHz triangle wave

Path length Time Delay (microsec)
75.0 m ..................... 0.2
36.5 m ..................... 0.1
24.5 m ..................... 0.05



c = 3.4 x 10^8 m/s

Modulation: 1.4 MHz triangle wave

Path length .......... Time Delay (microsec)
...........................front of wave..... back of wave
60.36 m ..................... 1.9 ..................... 1.1
40.12 m ..................... 1.8 ..................... 1.8
20.06 m ..................... 1.7 ..................... 0.85

Using the slope of the line gives c = 2.0 x 10^8 m/s

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