Radiation, Quanta, and Photons

The reason why classical mechanics is inadequate at the sub-atomic level is because it ignores quantum effects. Not only matter, but also radiation is discontinuous at the sub-microscopic level. Matter occurs as separate atoms and radiation as small packets, quanta or photons. At this level the two descriptions become identical - matter behaves like radiation and radiation behaves like matter. This is appropriate since matter and energy are interconvertible according to the relativistic relationship, E = me2. Wave and particle theories must therefore merge at the quantum level. 

A full explanation of the properties of light requires both the wave theory of electromagnetic radiation and the quantum theory. Most photochemical processes are best understood in terms of the quantum theory, which says that light is made up of discrete particles called quanta or photons. Each quantum carries an amount of energy, S, determined by the wavelength of the light, A. Equation 13.1, in which h is Planck s constant and c is the speed of light in a vacuum,... 

Electromagnetic radiation can be described as a wave occurring simultaneously in electrical and magnetic fields. It can also be described as if it consisted of particles called quanta or photons. 

The nature of electromagnetic radiation was described in terms of quanta or photons, and evidence for such a description is given from Planck s explanation of cavity radiation and the photoelectric effect. 

Unlike photochemistry that acts on a substance by using only photons of a relatively low, some 1—10 eV energy, the radiation chemistry makes use of high energy particles (10 to eV). These comprise y-quanta, fast electrons, fast nuclei, protons, deuterons, tritons, a-particles, fission fragments and neutrons. (For a detailed description of ionizing radiation sources and of their use in radiation-chemical studies see [60, 502].)... 

This classical description of electromagnetic radiation satisfactorily explained the properties of visible radiation in the ultraviolet region. According to equations derived for the behavior of electromagnetic radiation based upon the concept of continuous wave motion, the energy emitted by a blackbody source at finite temperature increases without limit as the wavelength of the radiation approaches zero. This behavior is not experimentally observed. However, if one assumes, as did Planck (1901), that radiation is emitted by a source in discrete units called quanta or photons and not continuously, then the behavior of radiation in the ultraviolet region can be satisfactorily explained. This concept of... 

We consider here excitation by high intensity radiation with many photons (quanta) per quantum field mode which can for all practical purposes be described as a classical electromagnetic wave satisfying the general wave equations (2) and (3) ... 

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