Classical mechanics light properties

We should note that the birth of the quantum theory came about in trying to explain the electronic structure of atoms and the properties of light. It became apparent toward the end of the nineteenth century that the classical laws of physics (classical mechanics as proposed by Isaac Newton in the seventeenth century) could not be used to describe electronic structure. The new theory of quantum mechanics, developed at the beginning of the twentieth century, was a scientific breakthrough that changed the way we view atoms. 

Basically, Newtonian mechanics worked well for problems involving terrestrial and even celestial bodies, providing rational and quantifiable relationships between mass, velocity, acceleration, and force. However, in the realm of optics and electricity, numerous observations seemed to defy Newtonian laws. Phenomena such as diffraction and interference could only be explained if light had both particle and wave properties. Indeed, particles such as electrons and x-rays appeared to have both discrete energy states and momentum, properties similar to those of light. None of the classical, or Newtonian, laws could account for such behavior, and such inadequacies led scientists to search for new concepts in the consideration of the nature of reahty. 

In the late nineteenth century, a whole set of experiments progressively lead to the conclusion that classical physics, namely, Newtonian mechanics, thermodynamics, and nascent electromagnetism, were unable to explain empirical evidence gathered by experimentalists. Scientists of that time were unable to conciliate two apparent contradictory aspects exhibited by radiation and matter. Some experiments demonstrated that light behaved like a wave, while others showed a rather corpuscular nature. On the other hand, electrons, protons, and the other massive particles would manifest wave-like properties in certain experimental conditions. 

Photophysics and photochemistry are relatively young sciences, a real understanding of light-induced processes going back some 50 or 60 years. The development of quantum mechanics was an essential step, as classical physics cannot account for the properties of excited states of atoms and molecules. In the past 30 years the advent of new experimental techniques has given a major impetus to research in new areas of photochemistry, and these are the subject of this final chapter. It must of course be realized that these developments advance all the time, and that we talk here of a moving frontier, as it is in 1992. 

Spin is typically treated as a quantum phenomenon an easily accesible and readable account is given by Ohanian [107]. However, the possibility that spin may be a phenomenon with classical overtones has been a recurrent one [79,107-111,122-124], The connection between the classical polarization of light and quantum mechanics was noted long ago by Fano [125], while the connection between polarization and Clifford algebra for spinors was noted more recently [126], Finally, some philosophers have suggested that spin is a mere property of space [127],... 

This chapter introduces the core concepts of what is now called classical physics (mechanics, electricity, magnetism, and properties of waves). Today we think of classical physics as a special case in a more general framework which would include relativistic effects (for particles with velocities which approach the speed of light) and quantum effects, which are needed for a complete description of atomic behavior. Nonetheless, we will find that this classical perspective (with a few minor corrections) serves as an excellent starting point for understanding many atomic and molecular properties. 

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