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Optics
Fibroptonic
Ref : 202 012
3 Experiment no. 18
Determining the period of a mechanical vibrator
4 Experiment no. 19
Vibrations of a loudspeaker diaphragm
5 Experiment no. 20
Oscillations of a spring. Measuring the period.
Figure 23: Set-up to determine the period of a spring.
ENGLISH
•
Repeat the previous set-up and replace the tuning fork with Melde's vibrator or a
saw blade; the small adhesive mirror will be fixed to the vibrator.
•
We thus show that the frequency of a Melde's vibrator is 100 Hz when it is
powered by the mains AC current (220 V – 50 Hz).
•
We can also check that the frequency of the fundamental mode (the one that is
generally driven) varies as the inverse square of the length of the vibrating blade.
In this case the excitation is not sustained.
•
Repeat the set-up of figure 22 and replace the tuning fork by a loudspeaker
whose visible membrane is of a large diameter (for example. 10 cm). Fix the
small adhesive mirror mid way between the centre and the edge of the
membrane.
•
Drive the loudspeaker with a low frequency generator, which gives a sinusoidal
signal. Adjust the frequency so that the amplitude of vibrations of the adhesive
mirror is significant.
•
The signal detected by the photoresistor module and viewed on the oscilloscope
is periodic. Measure its period and compare it with the indications of the low
frequency generator.
•
Use another loudspeaker fitted with a mirror, which also intercepts the laser
beam. It is now possible to obtain Lissajous figures by carefully combining the 2
periodic functions. Another LF generator will be required.
•
Project the laser on the photoresistor and intercept the beam with the small mass
hanging at the end of the spring (figure 23).
•
Adjust the amplitude of the signal detected by the oscilloscope using the
potentiometer on the photoresistor module (oscilloscope in AC).
•
Make the spring oscillate.
photoresistor
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