Download Lab 1 Oscilloscope Waves and Speed of Light

Physics 197, Fall 2015, Miramar College, Laboratory 1: Oscilloscope Waves and Speed of Light
Measurement.
1A: Investigate Waves on an Oscilloscope. (Work in Pairs)
1. Plug in RIGOL DS1102E 100 MHz Digital Oscilloscope and BK PRECISION 4011A 5 MHz Function
Generator, and turn on power.
2. Connect BNC to Alligator Cable to Output of Function Generator
3. Connect Oscilloscope Probe to Alligator outputs and CH 1 Oscilloscope input. (Black (GND) to black
lead and probe clip to red lead).
4. Set function generator on Sine Wave Pattern and 5 Hz range, and turn frequency knobs to get a
frequency of around 2 Hz (It takes a while for display to adjust).
5. Set time scale (horizontal) on Oscilloscope to 500 ms/division and voltage scale (vertical) to 500
mV/division. Adjust vertical offset to see whole wave, and horizontal offset so the refresh starts at the
left end. Press Run/Stop to stop (red) and then start (green) data measurement.
What do you observe?
6. Rotate the Coarse Frequency knob on the function generator to increase the frequency to about 5 Hz.
What changed on the oscilloscope display?
7. Change the horizontal time scale on the oscilloscope to 50 ms/division.
What do you observe?
8. Increase the frequency range on the function generator to the 5KHz range, and change the time scale
on the oscilloscope to 100 s per division. If the display is moving back and forth on the horizontal
scale, press AUTO on the oscilloscope (and adjust the trigger voltage if necessary) to stabilize the
display. The oscilloscope should now also be measuring and displaying the frequency and period of the
sine wave.
What is the measured frequency and period? How are they related?
9. Adjust the coarse frequency on the function generator back and forth.
What do you observe on the oscilloscope?
Write down some sets of measured frequency and period. How are they related?
10. Spend 10-15 minutes experimenting with other frequencies, other waveforms (square and triangle),
and other oscilloscope time scales.
Take notes about what you changed and what you observed in your lab notebook, including sketches of
oscilloscope traces.
11. Increase the frequency to 5 MHz, change the oscilloscope time scale to 50 ns/division, and observe
a square waveform.
At this high frequency, does the waveform still look square on the oscilloscope? If not, what do you
think is going on?
1B: Measure Speed of Light (2 groups, this is a new experiment with limited equipment. The instructor
will help).
For this experiment, we are going to modulate the light from a laser at about 3 MHz, and observe the
light reflected onto a photodetector as the path length from the laser to detector is changed. The
experimental setup and experiment which follow are from the PASCO “Laser Speed of Light System”,
model AP-8586.
1. Read through the Laser Speed of Light write-up from PASCO.
PRELAB QUESTION: What should you never do with a laser such as the one used in the experiment?
2. Set up the laser, detector and focusing lens on the rail using the component carriers. Connect the
Male BNC to Male BNC cable from the Square wave output of the function generator to Channel 1 of the
Oscilloscope. Connect the RCA to BNC cable from the Video output of the receiver to Channel 2 of the
Oscilloscope. Connect the BNC to phone plug cable from the output of the Function Generator to the
Laser.
3. Turn the DC Offset knob on the Function Generator all the way counterclockwise, and the Output
knob all the way clockwise. A 3 MHz square wave modulation seems to work best. Turn the laser on.
Press the DC Offset button on the Function Generator. Turn up the DC Offset until you see light coming
out of the laser.
4. Have the laser coming out as horizontal as possible (relative to the floor). Point the laser to a mirror
on a stand in the corner of the room, and adjust the mirror so that light is coming back into the detector.
Adjust the 127 mm focal length lens so that the light is focused at the center of the detector. Make sure
the photodetector is turned on.
Sketch your experimental set-up, labeling the different parts.
5. Set up the oscilloscope so it is Dual Trace, with Channel 1 about 1 V/division DC and Channel 2 about
500 mV/division AC. Adjust the DC offset on the function generator to get the maximum signal from the
photodetector on the oscilloscope.
Have the instructor explain something about how the diode laser is being modulated, and why the DC
offset shouldn’t just be turned to maximum to get the largest modulation signal. Write down an
explanation of the diode laser modulation in your lab notebook.
Discuss with the instructor (as a class) what we should expect to see when the path distance between
the laser/detector bench and the turning mirror is changed, and how we can use measurements of what
happens on the oscilloscope to measure the speed of light. Write down in your laboratory notebook an
explanation of how we will measure the speed of light.
6. Set a vertical cursor on the oscilloscope on a repeatable feature of the modulated signal from the
detector such as where the voltage crosses zero (AC scale).
7. Change the path distance so the laser crosses the whole room and comes back before hitting the
detector. Measure the change in path distance from the geometry in paragraph 4 (each floor tile is 1
foot long) and set a second cursor on the oscilloscope on the new location of the repeatable feature
used before. Measure the time difference. It is important that the signal on the oscilloscope looks very
similar to how it looked for the shorter path difference.
Record your measurements and any notes about them in your laboratory notebook.
8. Change the path distance so the laser crosses the whole room twice and comes back before hitting
the detector. Measure the change in path distance from the geometry in paragraph 4, and measure the
new time difference.
Record your measurements and any notes about them in your laboratory notebook.
Calculate the speed of light using your measurements, including a rough graph and your procedure.
9. If you think that something about the experiment is not working right, make changes until you are
comfortable that you are able to make a good speed of light measurement using the apparatus.
Discuss any changes made and how the results were affected. Discuss what modifications you would
make to the experiment to get a more accurate measurement.