Contrast theory

As z changes, the phases of these two rays change at different rates, so that they will alternate between constructive and destructive interference. The phase (f Q of the geometrically reflected normal ray is 

If the output of the transducer is detected by a phase-insensitive circuit (as is usually the case), then it is the difference between the phases of the two rays that is important. This is 

As the specimen is moved towards the lens, the two rays will alternate between being in phase and being out of phase. The period Az of the resulting oscillations in V(z) is the movement in the z-direction needed for a change of 2n in the relative phase, 

If this is expressed in terms of the wavelength in water, rather than the wavenumber, it becomes 

A derivation analogous to that of (7.7) gives the change in the total attenuation suffered by the Rayleigh ray, 

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At present, there are two contrasting theories serving to describe the dynamics of proton transfer when an electronic barrier exists in the transfer coordinate. The... 

Anyone who has successfully used a microscope to image properties to which it is sensitive will sooner or later find himself wanting to be able to measure those properties with the spatial resolution which that microscope affords. Since an acoustic microscope images the elastic properties of a specimen, it must be possible to use it to measure elastic properties both as a measurement technique in its own right and also in order to interpret quantitatively the contrast in images. It emerged from contrast theory that the form of V(z) could be calculated from the reflectance function of a specimen, and also that the periodicity and decay of oscillations in V(z) can be directly related to the velocity and attenuation of Rayleigh waves. Both of these observations can be inverted in order to deduce elastic properties from measured V(z). 

The transmitted field to the right of the crack can be calculated similarly, with a change of sign in eqn (12.38) and in the x-dependence of wc(x, () and wsc(x). In this way approximate values of 1r and Rr can be found for the two-dimensional crack contrast theory, and possibly for the three-dimensional theory as well. The calculated field is reasonably good near the free surface but not near the crack tip, so the approximations are better the deeper the crack is compared with the wavelength. 

The normal way to build a house is to acquire a piece of land on planet Earth, and then according to a master plan, to dig and make a foundation, to anchor in this foundation the solid structure allowing to place the walls and floors and at the end to cover the house with a roof. In contrast, theory and virtual reality allow us to proceed in reverse order, starting with a roof somewhere in cosmos and looking for ways to accommodate under it interesting structures that become stable when we add a planet with its gravitational field. 

Four main theoretical perspectives can be identified as being useful but partial theories of child language acquisition. These are semantic feature hypothesis prototype theory functional core concept theory and lexical contrast theory. All four perspectives rely on empirical research of children s lexical and semantic errors and attempt to identify the processes through which children attach meaning to lexical representation. 

A modem chemist should know that there are two ways of describing electronic stmcture... [that are] not two contrasting theories, but rather two representations or two guises of the same reality. Their capabilities and insights into chemical problems are complementary ... 

TEM diffraction contrast imaging is by far the most important technique for studying defects in great detail. From the diffraction contrast theory [3], defects introduce an extra phase to the electron wave, a = 2jtg-R, where g is the diffraction... 

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