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01_speed_of_light

01_speed_of_light - Modern Physics(Spring 2008 Laboratory...

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Modern Physics (Spring 2008) Laboratory session #1 Measurement of the Speed of Light (c) Preparation: Read this handout prior to going to lab and complete the pre-lab exercise. Objectives: Identify possible sources of systematic and random uncertainty. Estimate the magnitude of these uncertainties and judge their significance to a measurement. Identify the physical parameters which determine the speed of light in a given reference frame and a given medium. I. Introduction Physicists view the speed of light to be one of the fundamental constants of the physical universe. The demonstrated invariance of c to the motion of observers, the assumed constancy of c throughout the lifetime of the universe, and the presumed invariance of c to direction of propagation and location in the universe are but a few indicators of its universality and importance. As an invariant quantity, c serves as the cornerstone of special relativity, and it appears in many theoretical expressions of physics along with other fundamental constants such as e, h, and m e . Since c is now defined as 8 2.99792458 10 / c m s = , it might seem more appropriate for us to use c along with some time-of-flight intervals t to make distance measurements (such as the length of the laboratory). Alas, in Physics 130 we persist in re-measuring c , mainly to recognize and underscore its importance and to expose students to some modern instrumentation. Prior to the international decision to define c , there existed a 300-year history of improving measurements of c . While our present measurement permits an experimental certainty of only about 99.5% (an uncertainty of about 0.5%), it does spare the student from using lanterns atop snow-covered hills as Galileo attempted before the age of pulsed lasers and digital electronics.
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