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Al Tobias (wat4y) - Office: Gibson S123 & Physics 218, (434) 924-0538

Physics Demo Manual

Demonstrations are cataloged according to PIRA Bibliography


Due to Physics Building renovations, the lead time to set up demo requests has increased due to the need to transport equipment across campus. Please be kind and let me know well ahead of time what you need.

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 Geometrical Optics
 Speed of Light
 1/r2 Demonstrator
  video  - Ray Optics Kit
 Confocal Mirrors
 Concave Mirror
  video  - Light The Match
 Disappearing Beaker
 String Model of Refraction
 Total Internal Reflection
 Steal the Light
 Light Pipe and Fiber Optics
 Water Light Pipe
 Thin Lenses with Objects
 The Eye

Speed of Light


A direct measurement of the speed of light in air by measurement of the time delay of a 10ns pulse which travels two different paths after being split.


With the help of a student, measure the distance between the two mirrors as accurately as possible. The mirrors will be aligned before class and if disturbed, re-alignment can take several minutes depending on familiarity with the equipment. Make sure the oscilloscope is being projected for the class to see and plug in the laser diode. The first mirror intersects the beam at its edge and so only reflects part of the beam back to the detector. The rest of the beam travels to the end of the bench and back to the same detector. Two pulses should appear on the display. A somewhat careful measurement of the time between pulses and the distance between the mirrors can lead to a calculated value of the speed of light within 3% of the actual value.


  • Set the trigger holdoff to greater than the time between first and second pulse ie. 50 to 100ns.
  • Note that the laser source puts out 10ns pulses at the rate of 885kHz (1.13us between source pulses).


  • Oscilloscope
  • Tape Measure