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Optics
Fibroptonic
Ref : 202 012
1 Purpose
To determine the period of a vibrator or mechanical oscillator using a photoresistor module.
To use a photoresistor to study periodic or pseudo-periodic mechanical movements.
THE EXPERIMENTS DESCRIBED IN THIS PART ARE EASY TO PERFORM.
2 Experiment no. 17
Determining the period of vibration of a tuning fork
ENGLISH
MECHANICAL VIBRATIONS AND
FIBROPTONIC
1.1 Equipment:
•
Photoresistor module
•
6, 9 or 12 V DC power supply
•
Oscilloscope
•
Helium-neon laser
•
Small piece of mirror measuring a few square millimetres on a adhesive foil (mirror
splinters are very convenient).
•
Miscellaneous:
o
. Tuning fork
. Melde's vibrator
o
o
. Saw blade
o
. Spring
o
. Loudspeaker of about 10 cm diameter (recovery).
o
. Simple pendulum consisting of a ball attached to a thread
o
. LFG to drive the loudspeaker.
1.2 Set-up:
•
Power the photoresistor with 6, 9 or 12 V DC (black terminals connected to the
minus of the power supply and the green to the positive)
•
Connect the oscilloscope between the yellow and black terminals of the
photoresistor (black to the ground and yellow to channel A).
•
Fix the small adhesive mirror on one of the branches of the tuning fork (not useful
if the metal is fairly reflecting)
•
Project the laser beam on this mirror and intercept the reflected beam with the
photoresistor (figure 22). Strike the tuning fork.
•
Adjust the amplitude of the signal detected by the oscilloscope with the
potentiometer on the photoresistor module (oscilloscope selector switch on AC).
•
Strike the tuning fork again and determine its period on the oscilloscope. Deduce
the frequency (440 Hz for "A 3" tuning fork). Note that the signal transmitted by a
tuning fork that is not sustained is pseudo-periodic (and not periodic).
OSCILLATIONS
63
SHEET
09
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