Newbury Park High School - Physics

Experiment: Speed of Light

Purpose: Measure the speed of light, and compare it to accepted values.

Procedure:

Part I: Mirror Close to Laser

1.  Set up the equipment as shown on the diagram. (Mostly it should already be set up for you). Omit the lens for this part.

2.  Turn on the oscilloscope, the power supply, the interface box, and the laser.

3.  Adjust the laser beam to hit the mirror on the tripod and reflect back into the photocell

opening in the front of the interface box so that you get a presentation on the oscilloscope as shown to the right.

4.  Repeatedly press the button labeled “time base” until you get a presentation as shown in fig. 2. You may also have to adjust the wave height using the “wave height adjust knob”.

5.  Press the “Cursor button” two times until you get a set of two vertical dashed lines on the oscilloscope screen which should be labeled “DTA----“). Using the “cursor location knob” and the “choose cursor button” set the left cursor on the oscilloscope to the peak of the first wave. Once this is set, don’t move it during the experiment.

Part II: Mirror Far from Laser

1.  Move the mirror across the breezeway and set it up about 10-15 feet outside our back door. Please be fairly quiet and don’t disturb other classes.

2.  Adjust the laser beam so that it hits the mirror in its center.

3.  Adjust the mirror; using at first the tripod adjustments, then the fine adjustment screws on the back of the mirror, so that the laser beam hits the lens near the control box.

4.  Adjust the lens to direct the focused beam onto the photocell opening and obtain a good wave presentation on the oscilloscope. This will take some doing and is the heart of this experiment.

5.  Change the cursor on the oscilloscope and adjust the second cursor to the new peak location on the oscilloscope. Be sure you don’t move the first cursor setting from part I.

6.  Read the time between the cursors on the top of the screen. (ms = 10-6 sec)

7.  Measure the distance to the far mirror using the 100m tape measure. Be sure to double the distance (the beams goes and comes back)

  1. Divide the distance by the time to get the speed of light. The accepted value is 3.00 x 108 m/sec.

Equipment Set Up

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