Concepts of Light

Since photochemistry is essentially the study of electronically excited species, it is now necessary to see how light can alter the electronic configuration in an absorbing species, or how a change in the configuration can lead to emission of light. 

Maxwell s electromagnetic field theory describes radiation in terms of oscillating electric and magnetic fields. It is one or other of these fields (usually the electric one) that interacts with the electrons of the chemical species absorbing the radiation. 

The most significant subsequent modification of Maxwell s nineteenth-century picture of electromagnetic radiation is our awareness that wave motion may have particulate properties associated with it. Planck developed his the- 

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The wave interpretation of the interference pattern observed in Young s experiment is inconsistent with the particle or photon concept of light as required by Einstein s explanation of the photoelectric effect. If the monochromatic beam of light consists of a stream of individual photons, then each photon presumably must pass through either slit A or slit B. To test this assertion, detectors are placed directly behind slits A and B and both slits are opened. The light beam used is of such low intensity that only one photon at a time is emitted by S. In this situation each photon is recorded by either one detector or the other, never by both at once. Half of the photons are observed to pass through slit A, half through slit B in random order. This result is consistent with particle behavior. 

Albert Einstein s 1905 work on the photoelectric effect paved the way for one of the greatest advances of twentieth-century science, the theory of quantum mechanics. Light had always been regarded as a wave. Quantum mechanics introduced the concept of light being transmitted in wave packets, or photons, that have particle-like qualities as well as wave-like qualities. 

The concept of light addressable potentiometric sensors (LAPS) was introduced in 1988 [67], LAPS is a semiconductor-based sensor with either electrolyte-insulator-semiconductor (EIS) or metal-insulator-semiconductor (MIS) structure, respectively. Figure 4.13 illustrates a schematic representation of a typical LAPS with EIS structure. A semiconductor substrate (silicone) is covered with an insulator (Si02). A sensing ion-selective layer, for instance, pH-sensitive S3N4, is deposited on top of the insulator. The whole assembly is placed in contact with the sample solution. 

At this point it is appropriate to return to the general wave and particle concepts of light. To be more specific, we can ask whether an individual photon can be treated both as a plane wave and as a particle, and whether a broad light beam can be considered both as a stream of individual photon particles and as a plane wave. For a closer examination of these questions it becomes necessary to investigate every particular case with respect to the following points ... 

Quantum mechanics had exploded between 1923 and 1927. A. H. Compton, in 1923, had discovered the change in frequency of X-rays scattered from the electrons (the Compton effect).25 Compton and, independently, Debye had underlined the importance of this discovery in support of the Einstein conception of light-quanta or photon propagation in space.26... 

Use the concept of light wave reflection to explain why color value changes when a solution is diluted. 

The student is first introduced to the wave and particle concepts of light. Though minimal emphasis is placed on mathematical equations, the relationship between velocity, wavelength and frequency, as well as the correlation of energy to frequency, is described. Those regions of the electromagnetic spectrum that are most useful and convenient for characterizing chemical substances are emphasized. 

In addition, it ought to be kept in mind, considering the classical concept of light absorption by molecules, that, owing to the inertia of molecules, the light field is not capable of turning the molecule and thus excites the vibrations of the dipole oscillator only to the extent to which... 

From a functional point of view, this important property can be readily built into low molecular weight chromophore assemblies acting as artificial reaction centers (coordination compounds, the population of CT states is directly related to the concept of light-induced charge separation in photosynthesis. Whenever such CT states are photoreactive and lead to the formation of the same kind of permanent redox products as observed in photosynthesis, the most essential features of the primary light reactions have been successfully duplicated. In a more strict sense, this is of course only true, if actinic red or NIR-light of comparable wavelength is absorbed by both the natural and artificial photosynthetic systems. 

Scheme 1 The Concept of Light-Directed, Spatially Addressable Parallel SynthesisI " ...

The concept of light stabilization preventing biological defacement is unique and differs dramatically from the classic approach of dispersion of toxicants throughout a polymer to provide a biologically-free surface. Incorporation of a toxicant into a polymer may provide a biologically-free surface but it does not address the inevitable process of photooxidation. 

In recent years, the concept of light construction has led to the development of hybrid structural parts. Using parts made of pure plastic runs into repeated difficulties because of the material s low modulus of elasticity, since the only way to compensate for this drawback is to use much thicker structures. Combinations of sheet metal materials with plastics facilitate new solutions that do justice to the concept of lightweight construction and provide new properties at the same time (sound insulation, thermal conductivity, dynamic stability, corrosion protection). The precondition for successful development of such hybrid structural parts is a reliable bond between the two materials. In certain cases, this must be realized by adhesives that are flexible with regard to the a problem (different heat expansion coefficients of the materials), but still provide high strength levels. 

We now give some examples of how our research programme has been applied in the area of geometrical optics we begin with the domain-specific theory. On the basis of our understanding of the scientific content and of the research literature on pupils conceptions of light and its properties (see e.g. Driver et al., 1994, pp. 41-45 and 128-133), this is how the theory is formulated ... 

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