Week two of Modern lab was spent measuring the speed of light.  We are able to report a speed of 28.9 cm/ns or 2.89 times ten to the eighth power in meters per second.  Pretty good. 

Here is how we made the measurement.  We took a medium speed arbitrary wave (AWG) for generator (Rigol) and used the pulse feature.  We set the period to 500 ns, the pulse with to roughly 40 ns, and output voltage of 8 V (note it is reversed polarity so the leads to the diode are switched).  The diode fits into a socket which has its leads soldered to a wire that goes to the AWG.  The laser light passes through a collimation tube and is sent down a long path to a mirror where it is reflected into a fast photodiode. It is only on when the voltage is high.  Both the voltage controlling the laser and and photodiode signal is measured by a 200 MHz oscilloscope.  The delay time between each of these pulses is measured. The speed of light is the slope of the distance vs. delay time curve. This can be obtained by repeating this experiment over and over with different path lengths.

Here are some take aways:

• We can just barely do the experiment with the Moku:go.  The output power is limited to 5V.  You can get out a small amount of laser light out with that signal.  While I was able to make it work over the summer we did not try it with the students.  We will work on playing with the impendence of the output to the laser and/or giving our signal a DC offset.  Since we had the AWG and oscilloscope laying around we used them.  However, using the Moku:go could make this experiment inexpensive and mobile – thus increasing its usefulness.
• In the past we have done this experiment with the usual 3 MHz function generators we use in the intro physics classes. Here is my advice: Don’t use a 3 MHz function generator to do this experiment (e.g., pulse width near 150 ns).  Yes it is fun to send the pulses down the hall, but the rule of thumb is 1 ft for ns so you need go a distance on the order of the pulse (150 ns) to see a shift.  30-40 ns worked wonderfully.  The AWG was easy. I did borrow it from my research lab, but it is also pretty inexpensive. Next time, let’s try to get that pulse down to 10 ns!
• I need to teach uncertainty for next lab.  We need more than just a number. When the students just gave me a single number I cried inside. That is something we will talk about this week.

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