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
Diffraction
Interference
Color
Polarization
The Eye

### Speed of Light

#### Purpose:

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.

#### Procedure:

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.

#### Hints:

• 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