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Unformatted text preview: Chapter 3 Pre 19th Century Physics 3.1 Introduction We begin our study of modern physics by examining the phenomena as- sociated with light. Although the phenomena of light is among the oldest examined and there are theories of light that must go back to the first hu- mans, light is particularly interesting from the modern perspective because of the central role that it has played in the development of our ideas, partic- ularly quantum mechanics and relativity. To trace the development of our ideas about light from the ancients to today would take the entire semester and not allow any room for modern physics. For a concise coverage of these ancient ideas, the book by Park [Park 1997] is excellent. Instead, we will start with one of two threads of development that emerged in the 17 th cen- tury. In the 1660s, Fermat proposed that light travels between two points over the path that is the least travel time of all the possible paths, Sec- tion 3.2. At the time of its formulation, there was a competing theory: a particle theory often identified with Newton. The particulate theory was the generally accepted description, because it successfully accounted for all the phenomena known at that time to be related to light, basically reflection and refraction. Fermats approach equally well described the reflection and refraction experiments of the day. In a sense, there was a stalemate with the great prestige of Newton providing the edge to the particulate approach. A significant difference between the two theories was that Fermats Theory required that light traveled slower in a dense media whereas the particulate approach required that light traveled faster. At the time of formulation of these competing theories, it was impossible to measure the speed of light in 57 58 CHAPTER 3. PRE 19TH CENTURY PHYSICS dense media. Once the confirming experiment supported Fermats theory, it became the accepted approach. We now know that, in some sense, some of the aspects of the particulate theory are correct for a certain range of observation but we will get to this later in Chapter 7. With the measure- ment of the speed of light in dense matter, the particulate theory was then superseded by a Fermats theory. Fermats formulation was very successful in describing all the phenomena associated with light that was known at his time and, in fact, most of the common phenomena that we associate with light, see Section 3.3. As new phenomena, interference and diffraction, associated with light were observed, it became clear that a new construction was needed. Ex- tending and clarifying a construction associated with Huygens, a contem- porary of Newton, and Thomas Young, Fresnel formulated a new approach that in the appropriate limits reproduced to all the success of Fermat but incorporated the new phenomena of interference and diffraction, see Sec- tion 3.4. Integral to the success of this approach is the idea of an underlying continuous system, the ether, that was the basis for the the phenomena as-...
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- Spring '07