Optical diffraction by three-dimensional quasi-repetitive structures : some aspects of thick holograms

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Photonics in Nature: From Order to Disorder

The most vibrant and striking colours in living organisms are often caused by a combination of pigments and nano-scale transparent architectures, which interact with light to produce so-called structural colours. These colours are the result of light interfering with the nanoscale structures that are present in the materials. Such colour-producing structures are not perfect, and irregularities in the arrangements (disorder) are present in many organisms. However, disorder in natural structures is not detrimental but functional, as it allows a broader range of optical effects. This chapter reviews and attempts to classify structurally coloured organisms, highlighting the influence that disorder has on their visual appearance. It also showcases how photonic systems, such as the blue Morpho butterfly and the white Cyphochilus beetle, are capable of obtaining optical properties (long-distance visibility and whiteness, respectively) where disorder seems to be highly optimized, indicating that disorder is important for obtaining complex visual effects in natural systems. The chapter first introduces the mathematical concepts required for analysing disordered systems, such as the Fourier transform and the structure factor. Then, ordered and disordered natural photonic systems are reviewed. This is followed by examples of completely disordered structures responsible of white appearances. Finally, we review the possibilities of hierarchical organisation and pixelated surfaces to widen the range of optical appearances

Group representations arising from Lorentz conformal geometry

AbstractIt is shown that there exist conformally covariant differential operators D2l,k of all even orders 2l, on differential forms of all orders k, in the double cover n of the n-dimensional compactified Minkowski space n. These act as intertwining differential operators for natural representations of O(2, n), the conformal group of n. For even n, the resulting decompositions of differential form representations of O↑(2, n), the orthochronous conformal group, produce infinite families of unitary representations, the most interesting of which are carried by “positive mass-squared, positive frequency” quotients for 2l ⩾ ¦n − 2k¦. Physically, these generalize unitary representations of the conformal group associated with the modified wave operator D2,0 = □ + ((n − 2)2)2, and the Maxwell operator on vector potentials D2,(n − 2)2 = δd. All the representation spaces produced, unitary and nonunitary, may be viewed as infinite systems of harmonic oscillators. As a by-product of the spectral resolution of the D2l,k, one gets some striking wave propagative properties for all of the equations D2l,k Φ = 0, including Huygens' principle in the curved spacetime n. The operators D2l,k have not been seen before except in the special cases k = 0 or n, and k = (n ± 2)2, l = 1 (the Maxwell operator). Thus much new information is obtained even in the physical case n = 4

The life and work of Prof. George Chrystal (1851-1911)

This thesis is principally concerned with George Chrystal's life and his work, mainly in three directions viz., as an experimentalist, a mathematician, and an educationist. The main object is to bring to light the work of a personality who is representative of many more who are always forgotten. The majority of historians of science consider the works of the giants in science, ignoring totally the contributions made by the less prominent people like Prof. George Chrystal. In fact their contributions serve as one of the most important factors in propagation of scientific knowledge. His main contributions: verification of Ohm's Law experimentally; Non-Euclidean geometry; differential equations; text books on algebra; theory of seiches; institution of leaving certificate examination in Scottish education and many more have been discussed in detail. A survey of Chrystal's general thought is given in so far as it may be gathered from his scattered remarks. The references are mentioned by numerals in the superscript, details of which are given at the end of each chapter. The main text consists of six chapters. There are three appendices at the end,' Appendix 'A' consists of his correspondence with different scientists, most of which is still unpublished. Appendix 'B' contains a bibliography of his contributions in chronological order, and Appendix 'C contains his three Promoter's addresses. Tables and figures are attached at their proper places, including some rarely available photographs

The selectivity of human placenta in maternal-foetal transfer of plasma proteins : mechanisms and implications

The selectivity of human placenta in maternal-foetal transfer of plasma proteins : mechanisms and implication

Factors affecting brightness and colour vision under water

Both theoretical and practical importance can be attached to attempts to model human threshold and supra-threshold visual performance under water. Previously, emphasis has been given to the integration of visual data from experiments conducted in air with data of the physical specification of the underwater light field. However, too few underwater studies have been undertaken for the validity of this approach to be assessed. The present research therefore was concerned with the acquisition of such data. Four experiments were carried out: (a) to compare the predicted and obtained detection thresholds of achromatic targets, (b) to measure the relative recognition thresholds of coloured targets, (c) to compare the predicted and obtained supra-threshold appearance of coloured targets at various viewing distances and under different experimental instructions, (d) to compare the predicted and obtained detection thresholds for achromatic targets under realistic search conditions. Within each experiment, observers were tested on visual tasks in the field and in laboratory simulations. Physical specifications of targets and backgrounds were determined by photometry and spectroradiometry. The data confirmed that: (a) erroneous predictions of the detection threshold could occur when the contributions of absorption and scattering to the attenuation of light were not differentiated, (b) the successful replication of previous findings for the relative recognition thresholds of colours depended on the brightness of the targets, (c) the perceived change in target colour with increasing viewing distance was less than that measured physically, implying the presence of a colour constancy mechanism other than chromatic adaptation and simultaneous colour contrast; the degree of colour constancy also varied with the type of target and experimental instructions, (d) the successful prediction of the effects of target-observer motion and target location uncertainty required more than simple numerical corrections to the basic detection threshold model. It was concluded that further progress in underwater visibility modelling is possible provided that the tendency to oversimplify human visual performance is suppressed

1973-1974 Bulletin

Volume LXXXIV, Number 2 Scanned from the copy held in University Archives and Special Collections.https://ecommons.udayton.edu/bulletin/1061/thumbnail.jp

Advanced dielectrophoretic cell separation systems

This thesis describes experimental and theoretical investigations into new particle handling and separation methods and techniques. It makes a major contribution to the rapidly expanding field of cell separation technology. A novel dielectrophoretic cell separation system has been developed, which is capable of processing large sample volumes (~50mL) in a flow through system. Previously reported dielectrophoretic cell separator systems typically process sample volumes in the 100mL range. The electrode configuration developed for this work allows the isolation and concentration of single particle types from large sample volumes; a method which could be further developed into a new rare-cell separation technology. In addition, a new technique of particle fractionation was developed termed ‘Dielectrophoretic Chromatography’. A cell separation chip was designed and built using standard micro-fabrication techniques. Experimental work was undertaken to demonstrate the function and limitations of the device. Numerical modelling of the particle motion in the device is presented and compared with experimental work for a number of different particle types, applied voltages and fluid flow rates. The dielectrophoretic separation system comprises a microfluidic channel, of cross-section 100mm x 10mm and length 50mm, with two sets of interdigitated microelectrode arrays. The first set of arrays, with characteristic electrode size 40mm, called a focussing device, has electrodes patterned onto the top and bottom surfaces of the flow channel. The second electrode array, which is part of the same device, has an electrode array patterned only on the bottom of the channel. Two sizes of secondary electrode array were used 20mm and 40mm. AC voltages (from 1V to 10V peak) are applied to the microelectrode, with a frequency between 10kHz to 180MHz. A dielectrophoretic force is exerted on the particles as they flow along the channel. The first electrode array uses negative dielectrophoresis to focus the stream of particles entering the device into a narrow sheet (one particle diameter thick) midway between the upper and lower channel surfaces. The second electrode array, down stream from the first is separately controllable