8/26/02clc

 

Speed of Light – pulsed N2 laser

Note: BEFORE DOING THIS EXPERIMENT YOU WILL BE REQUIRED TO SIGN A PAGE SHOWING THAT YOU HAVE READ AND UNDERSTAND THE SAFETY RULES FOR THIS EXPERIMENT AND THAT YOU WILL FOLLOW THEM.  IF YOU WANT TO READ THE RULES BEFORE YOU COME TO CLASS THEY ARE AT THIS ADDRESS.

 

 

      When Michelson did his classic experiment to measure the speed of light he “pulsed” the light by sending it off a rapidly rotating mirror. Technology has progressed since then, and your life will be made easier by using a pulsed laser beam instead. The source is a pulsed nitrogen laser operating at 337 nm at about 20 Hz with a pulse length near 10 ns. Part of the beam is deflected by a glass slide, after just a few centimeters, into a photodiode, while the main beam travels across the room and back into the same photodiode .  If you trigger an oscilloscope with the synchronization pulse from the source, both pulses appear on the trace and you can measure the time interval between them. By measuring this time difference, and the distance, you can calculate the speed of light. 

   A couple of tips: the laser is so strong you will not need collecting lenses to get enough light onto the diode. However, you should adjust the detector so that the beam from the glass slide, which can be quite intense, gives a  pulse height  approximately equal to that from the distant mirror. Be sure you have a 10 ns pulse, not a very long pulse, which would indicate saturation of the detector.  You will find that the oscilloscope has a built-in time-measuring capability, and you can check the calibration using the provided 1 kHz oscillator on the scope and a probe.

  Turn off both the laser and the photodiode preamp (small switch down) when you are not using them for some time. Both have finite lifetimes.

  Try to get the absolutely most accurate result you can. This will probably mean making enough independent measurements of the important variables, perhaps by different students, to evaluate averages and standard deviations for all measured quantities. What do you think is the limiting factor in the measurement of the speed of light in this case?  What “systematic” errors are there?

 

 

    WARNING: THIS IS A VERY POWERFUL LASER CAPABLE OF DOING SERIOUS AND PERMANENT DAMAGE TO YOUR EYES. Fortunately the radiation is in the ultraviolet and is severely attenuated by safety glasses. You can see it easily on a piece of paper, which it causes to fluoresce brightly in the blue. BEFORE TURNING ON THE LASER BE ABSOLUTELY SURE THAT EVERYBODY IN THE ROOM IS WEARING SAFETY GLASSES AND KEEPS THEM ON DURING THE WHOLE TIME THE LASER IS ON. BE SURE THE DOORS TO THE ROOM ARE CLOSED AND THE WARNING SIGNS ON THESE DOORS ARE IN PLACE. Even with the safety glasses on, be careful not to look directly into the laser beam during this experiment. Also do not expose your skin to the beam for any extended period. YOUR ONLY SAFEGUARD AGAINST ACCIDENTS IS YOUR OWN CARE. This is particularly important in this case because the height of the beam is such that it is fairly easy to look at directly. DO NOT DO THIS.

Your apparatus list includes:

 

 

Supplementary exercises:

 

1) Use the oscilloscope with readout capabilities to record some typical spectra for your report, as well as the scope trace for parts 2-4 below.

2) Using the pulse from the photodiode as a  “short pulse” and a long RG 62 cable, measure the effect of terminating the cable with 50 ohms, with a short circuit and with an open circuit.

3) Using this  pulse, measure the speed of electromagnetic waves in RG 62 cable.

4) Discuss your results.