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Optics
Fibroptonic
Ref : 202 012
Figure 20: Signal observed on the oscilloscope when the photoresistor is lit with a neon
indicator powered by mains current. The shape of the signal depends on the experimental
conditions.
6 Experiment no. 15
Variations in the brightness of a fluorescent lamp powered by mains current.
Figure 21: Signal corresponding to periodic fluctuations in brightness of a fluorescent lamp
powered by mains current. The shape of the signal depends on the experimental
conditions.
7 Experiment no. 16
Determining the rotational speed of the mechanical stroboscope disk and frequency
calibration.
ENGLISH
•
Repeat the previous experiment and illuminate the photoresistor with a single
fluorescent lamp (this gives a white light, its spectrum contains certain mercury
lines during transmission).
•
Figure 21 shows the shape of the signal detected on the oscilloscope. Note the
asymmetry of the signal between the lighting and quenching phase of the lamp.
•
The oscilloscope is in the AC position.
•
Illuminate the photoresistor with a pocket torch or a DC powered light source.
•
Intercept the beam, which lights the photoresistor with the rotating disk of a
mechanical stroboscope.
•
We observe a non-sinusoidal alternating signal whose appearance depends on the
rotational speed of the disk, on the number of slits or holes, on the width of these
openings, etc.
•
Even so the signal is periodic. If the disk has only one opening, the frequency
measured corresponds to the rotational speed of the motor in revolutions per
second. If the disk has 4 evenly spaced out identical openings, the frequency of
the flashes by the stroboscope is equal to 4 times the rotational speed of the motor.
•
We can thus use this experiment to determine the rotational speed of a motor.
Simply fit it with a disk with one opening.
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