Quantization, of light

The history is more complicated. Briefly, Planck proposed the quantization of light as a mathematical model, but it was Albert Einstein in 1905 who asserted its physical validity. The name photon was bestowed on these quanta in 1926 by G. N. Lewis, better known to general chemistry veterans for his dot structures. 

Although the quantization of light explained the photoelectric effect, the wave explanation of light continued to have explanatory power as well, depending on the circumstances of the particular observation. So the principle that slowly emerged (albeit with some measure of resistance) is what we now call the wave-particle duality of light. Sometimes light appears to behave like a wave, at other times like a particle. Which behavior you observe depends on the particular experiment. 

Statistical properties of light are described within the framework of quantum optics which is based on a quantized description of the electromagnetic field. In section 21.2 we will depict specific experimenfs which have been performed fo show fhaf a quanfum description is necessary in some cases. We will describe in Section 21.3 fhe sfandard fools for fhe analysis of fhe sfafisfical properties of lighf and give fhe resulfs obfained for a number of sources. 

We have shown in this chapter how some experiments made it necessary in some cases to use a quantum description of light instead of the standard semi-classical theory where only the atomic part is quantized. A brief description of different helds in terms of their statistical properties was also given. This description makes it possible to discriminate between the different sources using the intensity autocorrelation function (r). 

The model of metal-ammonia solutions that has emerged is based on ionization of the metal atoms to produce metal ions and electrons that are both solvated. The solvated electron is believed to reside in a cavity in ammonia, and thus it may behave as a particle in a three-dimensional box with quantized energy levels. Transitions between the energy levels may give rise to absorption of light and thereby cause the solutions to be colored. The dissolution process can be represented as... 

The amount of energy per particle of light is fixed. We say it is quantized. 

And each photon has a fixed energy E. We say the energy is quantized. The intensity of a beam of light is merely a function of the number of photons within it per unit time see below. 

Materials and substances are composed of particles such as molecules, atoms and ions, which in turn consist of much smaller particles of electrons, positrons and neutrons. In electrochemistry, we deal primarily with charged particles of ions and electrons in addition to neutral particles. The sizes and masses of ions are the same as those of atoms for relatively light lithiiun ions the radius is 6 x 10 m and the mass is 1.1 x 10" kg. In contrast, electrons are much smaller and much lighter them ions, being 1/1,000 to 1/10,000 times smaller (classical electron radius=2.8 x 10 m, electron mass = 9.1 x 10" kg). Due to the extremely small size and mass of electrons, the quantization of electrons is more pronoimced than that of ions. Note that the electric charge carried by an electron (e = -1.602 X 10 C) is conventionally used to define the elemental unit of electric charge. 

If the result go = e would come out of a pure theoretical deduction, then the electronic charge would no longer be an independent constant of nature, but would become a quantized charge determined by Planck s constant and the velocity constant c of light, as indicated by Eq. (44). According to relation (43), this would then also apply to the product Mqiuq, whereas all quatitities Mq and mo have thus far not been deduced theoretically for the electron, but have been determined by measurements. 

As the twentieth century progressed, it became increasingly clear that quantization was not only a characteristic of light, but also of the fundamental particles from which matter is constructed. Bound electrons in atoms, in particular, are clearly limited to discrete energies (levels) as indicated by their ultraviolet and visible line spectra. This phenomenon has no classical correspondence - in a classical system, obeying Newtonian mechanics, energy can vary continuously. 

A more particle-oriented consideration of light shows that light is quantized and is emitted, transmitted, and absorbed in discrete units, so-called photons or quanta. The energy E of a photon or quantum (the unit of light on a molecular level) is given by ... 

Absorption of light is fundamental to all aspects of photochemistry and provides the basis for absorption spectroscopy.3 5-11 Light absorption is always quantized. It can take place only when the energy hv of a quantum is equal to the difference in energy between two energy levels of the absorbing molecule (Eq. 23-4). 

Electron Motion Around the Nucleus. The first approach to a treatment of these problems was made by Niels Bohr in 1913 when he formulated and applied rules for quantization of electron motion around the nucleus. Bohr postulated states of motion of the electron, satisfying these quantum rules, as peculiarly stable. In fact, one of them would be really permanently stable and would represent the ground state of the atom, The others would be only approximately stable. Occasionally an atom would leave one such state for another and, in the process, would radiate light of a frequency proportional to the difference in energy between the two states. By this means, Bohr was able to account for the spectrum of atomic hydrogen in a spectacular way. Bohr s paper in 1913 may well be said to have set the course of atomic physics on its latest path. 

The photoelectric effect was originally described by Albert Einstein and helped to establish the quantized nature of light. The photoelectric effect has many extremely... 

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