Electromagnetic waves and photons

Classical physics remains an excellent approximation to much of the behaviour of bodies on a macroscopic scale. It is in the microscopic realm that the quantum theory is essential. The behaviour of electrons in atoms and molecules, and the nature of the chemical bond, are among the problems that classical physics is unable to describe. It was only following the development of the quantum theory that chemists could really use physical ideas to provide a satisfactory understanding of their own problems. 

The word quantum Is originally Latin meaning how much Its plural quanta. 

In all formulae in this book, the arguments of sin and cos functions are In radians, not degrees. 2jt radians is the same as 360 degrees, and gives one complete cycle of oscillation. 

and vare Greek letters, called phi, lambda, and nu, respectively. 

There are several reasons for starting this account with a discussion of electromagnetic radiation. Historically, it was in this area that the quantum theory first developed. It is easier here to understand the evidence for the theory, and to appreciate some of its paradoxical consequences, than it is in the quantum theory of matter. The applications of the light-quantum hypothesis, as it was first called, also provide key pieces of evidence for the quantization of energy in atoms and molecules. Studies of the absorption and emission of radiation—the field of spectroscopy—and of the effect of light on chemical reactions—photochemistry—are very important areas of modem chemistry, in which the quantum nature of radiation is crucial. 

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All the considerations that follow are only valid for radiation that is stimulated thermally. Radiation is released from all bodies and is dependent on their material properties and temperature. This is known as heat or thermal radiation. Two theories are available for the description of the emission, transfer and absorption of radiative energy the classical theory of electromagnetic waves and the quantum theory of photons. These theories are not exclusive of each other but instead supplement each other by the fact that each describes individual aspects of thermal radiation very well. 

Radiation Energy traveiiing in the form of electromagnetic waves or photons, or a stream of particles such as alpha- and beta-particles from a radioactive source or neutron from a nuclear reactor. 

You can appreciate why scientists were puzzled The results of some experiments (the photoelectric effect) compelled them to the view that electromagnetic radiation is particlelike. The results of other experiments (diffraction) compelled them equally firmly to the view that electromagnetic radiation is wavelike. Thus we are brought to the heart of modern physics. Experiments oblige us to accept the wave-particle duality of electromagnetic radiation, in which the concepts of waves and particles blend together. In the wave model, the intensity of the radiation is proportional to the square of the amplitude of the wave. In the particle model, intensity is proportional to the number of photons present at each instant. 

The transfer or conversion of energy is always associated with the emission of electromagnetic waves. We met this concept in its simplest form in Chapter 2, when we looked at the transfer of infrared radiation (i.e. heat). This emission of photons occurs because all objects contain electrically charged particles and, whenever an electrically charged particles accelerates, it emits electromagnetic waves. 

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