Fisica

PHYSICS 6 - The Nature of Light
Gary R. Goldstein (© 2005) The perception of light is the principal means by which we know the world. From the single celled creatures we see under a microscope, to the most distant stars seen in a telescope, it is light that informs us. The incredible variety of forms and the structure of our universe are revealed by light. Yet the nature of light eludedscientists and natural philosophers through most of recorded history. Only since the mid-nineteenth century have physicists begun to understand this most fundamental natural phenomenon. The understanding physicists have today required the development of electromagnetic theory, relativity, and quantum mechanics. Even with the full array of techniques of modern physics there remain unsolved problems havingto do with the interaction of light with matter. An accurate explanation of this phenomenon requires a considerable background in physics and mathematics. We assume most of you do not have that preparation. And even if you did, the development necessary for a complete exposition would fill the semester. So a qualitative explanation is what we will aim for in the following, hoping to provide a hintof the subtlety and beauty of this illuminating phenomenon. The amount of very abstract thinking required to obtain a qualitative understanding is formidable. You will have to do a lot of cogitating to follow this development, but if you do make that effort you will be rewarded with a significantly deeper appreciation of the natural and technological environment in which we live. ElectromagneticRadiation We’ve pointed out that light has wave-like properties. This was known by Huygens in the seventeenth century. But if light is a wave, what is waving? You know that a water wave is the propagation of a disturbance through the medium of water. A sound wave is the 'propagation of a displacement through the medium of a gas (or a liquid or solid as well). What is the medium through which lightwaves propagate even in outer space? What is the nature of the disturbance that is propagated? What provides the restoring force that causes the propagation? These questions were answered in 1864 by the most brilliant and profound thinker of nineteenth century physics, James Clerk Maxwell, who, with Newton and Einstein, has had the greatest impact on our understanding of the physical world and onthe development of modern technology. (It is unfortunate that the magnitude of Maxwell's achievements is not as widely-appreciated by the liberally educated as by the scientific community.) Maxwell arrived at his theory of light by his study of electricity and magnetism. Previously physicists had unsuccessfully sought an explanation of light waves as some kind of mechanical vibration of anethereal medium, called ether, which was presumed to permeate the universe. That the explanation was not mechanical but electromagnetic was probably as unexpected and astonishing as Newton's realization that the solar system is held together by the same forces that cause objects to fall to the ground. In both Newton’s and Maxwell’s work it was the attempt to unify seemingly unrelated phenomena that ledto their great achievements. Some background material is necessary in order to see what Maxwell unified. Electric and Magnetic Forces Sometime in your life you learned something about electric and magnetic forces. Two electrically charged objects repel or attract each other in proportion to the product of their charges. If the objects considered are points or charged spheres, the force isinversely proportional to the square of the separation. This force law – Coulomb’s Law - is responsible for the flow of current in a wire as well as the structure of atoms and molecules. It is universal in the sense that any charged object will have its motion determined by adding up the forces exerted on it by all other charged objects in space. The force law for magnetism was more specific in its...