New_SoL_experiment - M easurement of the Speed of Light...

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1 Measurement of the Speed of Light with a Pulsed Laser Background Attempts to measure the speed of light date back at least to Galileo during the Late Renaissance. In his experiment (reported in 1638), he flashed a lantern to a friend on a distant hill who responded by flashing his own lantern back as soon as he saw the light. Alas, the experiment failed because the time necessary for light to travel from one hill to the other was much shorter than the time it took for each person to perceive and respond to the flash. But the idea of Galileo’s—to measure the time between sending and receiving a flash of light over a known distance—is perfectly valid and indeed is the principle of this experiment. The apparatus used here replaces the persons used by Galileo with electronic circuits, which have much more rapid response times that can be much more precisely measured. The flash of light is created by a pulsed laser diode, a device very similar to a laser pointer, except this laser is switched on and off— pulsed —at a very high rate: 1 million times per second (1 MHz). The pulsed beam can be bounced off of a mirror and the return pulses detected by a very responsive (“fast”) detector. Each pulse of the laser creates two signals, one from the laser itself (the “trigger”) and one from the detector when it picks up the returning pulse. These two signals can be compared to each other with a fast oscilloscope, and the time between them can be measured as a function of the distance between the laser/detector apparatus and the mirror. By varying the total distance from the laser to the detector L and monitoring the time difference t on the scope, one can determine the speed of light c. If the time difference between the two signals t depended only on the time it took for the light to travel that distance, then c would be given by L c t . (1) Now, if you have studied much physics, you may be bothered by one fact: by common assent, since 1983 the speed of light (in vacuum) is defined to be 299,792,458 m/s. Thus, an experimental measurement of it seems out of line; the unit of distance, the meter, is taken to be 1/299,792,458 of the distance that light travels in exactly 1 second. So, how can we say we are honestly measuring the speed of light when we use the meter to express the distance traveled by light in this experiment? Indeed, what is the point of the exercise? These are fair questions, and they raise fundamental issues that are important in every experiment. First, any experimental design must confront the question of standards: how does one know that the experiment actually makes a good measurement? One of the most common ways to check is to measure a quantity that is already known, for example, a spectroscopist might check the hydrogen or mercury spectrum with her instrument to see that it gives the expected spectral lines before turning her attention to an unknown sample. Second, every real measurement has some uncertainty: there are limits to how well any measurement can be made.
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This note was uploaded on 01/20/2012 for the course PHYSICS 331 taught by Professor Wiaodongxu during the Fall '11 term at University of Washington.

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New_SoL_experiment - M easurement of the Speed of Light...

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