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        The velocity of light in free space is one of the most important and intriguing constants of nature.  Whether the light comes from a laser on a desk top of from a star that is hurtling away at fantastic speeds, if you measure the velocity of the light, you measure the same constant value.  In more precise terminology, the velocity of light is independent of the relative velocities of the light source and the observer.

    Furthermore, as Einstein first presented in his Special Theory of Relativity,  the speed of light is critically important in some suprising ways.  In particular:

1)    The velocity of light establishes an upper limit to the velocity that may be imparted to any object.

2)    Objects moving near the velocity of light follow a set of physical laws drastically different, not only from Newton's Law, but from the basic assumption of human instutition.

With this in mind, it;s not suprising that a great deal of time and effort has been invested in measuring the speed of light.  Some of the most accurate measurements were made by Albert Michelson between 1926 and 1929 using methods very similar to those you will be using with PASCO Speed of Light Apparatus.  Michelson measured the velocity of light in air to be 2.99712*10^8 m/sec. 

But Michelson was by no means the first to concern himself with this measurements.  His work was built on a history of ever-improving methodology.