6A01.10-Speed of Light

Description: This demonstration illustrates a method for measuring the speed of light.


  • Metrological Laser
  • Beam Splitter
  • Mirror
  • 60MHz Oscilloscope
  • Photodetector
  • 4 Banana Plug/BNC cables

Set up:

  1. Plug Banana/BNC Cables into their respective spots in the Photodetector and Oscilloscope
  2. Turn on laser and adjust it pointing away from the Photodetector (Two small holes on side of Photo Detector)
  3. Adjust the Beam Splitter such that the reflected beam is aimed at the hole that is closer to the edge of the Photodetector
  4. Next, place the Mirror a good distance away from the Beam Splitter and aim it at the second hole in the Photodetector.
  5. Turn the Oscilloscope on.


  1. Measure the relative distances of the components:
    • Laser to Beam Splitter
    • Beam Splitter to Mirror
    • Mirror to Photodetector
    • Beam Splitter to Photodetector
  2. Use these distances to calculate the distance the laser travels from when the first beam enters the first hole, to when the it reaches the second hole.
  3. On the Oscilloscope, the screen should display 2 sin waves. The phase difference of the two peaks will yield the time difference. (Set the time divisions on the Oscilloscope to 0.1s )
  4. Simply use v=d*t to find the velocity which should be the speed of light.

Concept: The light will leave the laser and travel through the beam splitter, one path to the photodetector and the other to the mirror, and the oscilloscope will measure the time it takes between when the light enters the first hole, to when it reaches the second. Then, one measures the distance that the light will travel in the time measured and calculate the speed of light.

TIPS: Different Oscilloscopes with different measurement frequency have different minimum distances for the best results. For example, the 60MHz Oscilloscope has a minimum distance the light will travel in the measured time of 5m.

Caution, Laser Laser Light

Don’t point laser at students, or anybody else for that matter.