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    The speed of light is one of the few true constants in Physics; no matter what reference reference frame, the speed is always the same. Light's velocity also represents the absolute speed thatanything can travel, so there has been enormous intrest in the actual value of speed of light.  Galileo first tried to measure it and figured that the speed of light was 2.1*10^8 m/sec using Jupiter and its moons.  Fizeau found the speed to be 3.15*10^8 m/sec using a cogwheel and mirrors.  Foucalt improved Fizeau's method by using a rotating mirror instead of a cogwheel.  His number for the speed of light was 2,99774*10^8 m/sec.  And It is his experiment which is done that shows the results.


The purpose of this lab is to use a system of lenses and mirrors to estimate a value for the speed of light.  The method to be used in this experiment is based on Foucalt's 1862 experiments.


1)    Obtain a 1 meter optical bench and place a 0.5 mW He-Ne laser at the meter (100 cm)  end and he high speed rotating mirror at the 0 meter end of the bench.  Place the 48 mm focal length lens at the 93.0 cm mark, the 252 mm focal length lens at the 62.2 cm mark.  Place the measuring microscope at the 82.0 cm mark.  Place polarizers in fornt of the laser so one can safely observe the laser beam.

2)    Place the reflecting mirror on the opposite side of the room.  Now simply adjust the mirror so that the return beam is exactly opposite of the incident beam.

3)    Turn on the rotating mirror and let it warm up.   Now push the "max rev/sec"  button.  Use the micrometer knob to align the cross hair on the centroid.  Record maximum revolutions and the micrometer reading.

4)    Stop the mirror and reserve and direction of the rotation and repeat step 3.