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Lecture 1

# PHYS 242 Lecture 1: PHYS242_Lecture_04-06

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School
Department
Physics
Course
PHYS 242
Professor
Wolfgang Rau
Semester
Fall

Description
13 ENPHPHYS 242 Fall 2015 L 04 1.2 Speed of Light 1.2.1 Historical Introduction To our everyday experience light seems to arrive from its source instantaneously. For a long time in history most people believed that this is actually true. The first documented attempt to disprove this notion was by Galilei, who used lanterns, shutters and two experimenters at a large distance, but he found no indication of a finite speed of light after he had taken the human reaction time into account and could only conclude that light was fast. The first know (more or less successful) determination of the speed of light was by Ole Rmer and Christian Huygens (167678) using Jupiter moon eclipses. The observation of Jupiter moons had been proposed as an exact tool to measure time, which was necessary to improve navigation. Rmer noticed systematic changes in the eclipse times: when Jupiter was moving away from Earth the period appeared longer as compared to times when the distance between Jupiter and Earth was decreasing. He proposed that the origin of this effect was the finite speed of light. Using these results Christian Huygens calculated the speed of light to be ~2.1 10 ms. One of the longstanding questions about light was if it behaves like particles in the sense that its velocity depends on the velocity of the emitting object (like the balls thrown from the boat in our earlier example) or like waves which would imply that there is a medium carrying the light and that its speed is always the same with respect to the medium. This question was very difficult to answer since it was impossible to accelerate a mechanical system in the laboratory to a speed anywhere close to the speed of light. Even today we typically reach only speeds of order of 10 ms e.g. in turbo molecular pumps or ultracentrifuges. Astronomical systems provide us with considerably higher speeds; typical values in our proximity are 10 10 ms (e.g. the Earth moves about Sun with ~30 kms; the Sun about the galaxy with ~230 kms). 1.2.2 Three Important Observations A) Binary Stars A binary star system is a system with two stars orbiting each other. If a binary star system is not too far away and the two stars are far enough apart they can be observed individually. Such a system can be used to decide the above question: if we assume that the light moves with a given speed (from now on we will always use as the symbol for the speed of light) relative to the emitter we can calculate the time the light has to travel to reach an observer on Earth. Let us assume an orbital velocity of for the two stars and a distance between the W. Rau
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