PASCO AP-8586 Laser Speed of Light Apparatus - Manual (Page 8)

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Model No. AP-8586 Laser Speed of Light System
7
WARNING: Never look directly at the laser light source or
reflected light, such as from a mirror. Although the laser used
in this experiment is of low power, looking directly into the
laser light source or its reflected light from a mirror could cause severe
eye injuries or burns. To avoid eye injury, do not look directly into the
beam of the laser and wear laser protective goggles. When aligning the
laser, use the alignment marker. For more information about laser
safety, see Appendix C of this manual.
14. Align the laser, mirror, lens and receiver so that the laser beam is
focused onto the “Video” sensing element of the receiver.
15. Set the scope to dual trace:
a) Set channel 1 to 1v/div., DC.
b) Set channel 2 to 1 v/div., AC.
c) Set the trigger to channel 1.
d) Set the trigger level to about 2.5 volts.
e) Set the time base to 50 ns/div.
16. Adjust the alignment of the laser, mirror, lens, and receiver to
maximize the sine wave signal on channel 2.
17. Adjust the DC offset and amplitude of the function generator to
maximize the signal on channel 2.
Experiment 1: Modulated Laser Method
for Measuring the Speed of Light
In this experiment, you will measure the speed of light using a laser
modulated at a very high frequency and an oscilloscope. You will
measure the time, , that elapses while the light signal travels a
known distance, , and you will calculate the speed of light, which is
defined as .
The light signal, originating at the laser, will travel to the mirror and
back to the light receiver. You will vary
by moving the mirror and
measuring the corresponding effect on with the oscilloscope.
It is not necessary (or practical) to measure the actual elapsed time,
since we are only interested in how
changes as varies.
Therefore, you will actually measure an elapsed time relative to an
∆t
∆d
∆d ∆t ⁄
∆d
∆t
∆t ∆d
∆t′
WARNING: Always
inform your students of
the hazards of lasers
and enforce appropriate
safety precautions in
your classroom. For
more information, see
Appendix C: Laser
Safety in this manual.