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Results for “reflection”

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At least 721 records · Page 40Linked to original sources

PubMed2005

Central obesity is associated with reduced peripheral wave reflection in Indigenous Australians irrespective of diabetes status.

OBJECTIVE: To determine the influence of central obesity and type 2 diabetes on peripheral wave reflection in Indigenous Australians. DESIGN AND METHODS: A cross-sectional study of remote Indigenous Australians with (n = 43) and without (n = 54) type 2 diabetes of similar age (47 years) and sex; using anthropometric and bioelectrical impedance measures of obesity and applanation tonometry to determine the aortic augmentation index (AI) as an index of peripheral wave reflection. RESULTS: Indices of obesity were significantly higher in the diabetic than non-diabetic participants [body mass index (BMI): 27.3 versus 24.6 kg/m, P = 0.018; waist circumference: women 101 versus 94 cm, P = 0.008, men 102 versus 91 cm, P = 0.039]. AI was negatively related to obesity: BMI (r = -0.35, P = 0.0003), weight (r = -0.44, P < 0.0005), waist circumference (r = -0.34, P = 0.0003) and fat mass (r = -0.35, P < 0.0005). There was no significant difference in AI between the groups with and without diabetes. On multiple regression analysis, 66% of the variance in AI was explained with the following significant predictors: age, heart rate, male gender, fat mass and mean arterial pressure. Similar results were obtained when weight, waist circumference or BMI were substituted for fat mass. CONCLUSION: When compared with Indigenous Australians without diabetes, those with type 2 diabetes do not have greater aortic pressure augmentation from peripheral wave reflection. However, obesity, irrespective of the index used, was related to lower peripheral wave reflection in both those with and without type 2 diabetes.

Anthropometry↗

PubMed2005

Migrating locusts can detect polarized reflections to avoid flying over the sea.

The desert locust Schistocerca gregaria is a well known migrating insect, travelling long distances in swarms containing millions of individuals. During November 2004, such a locust swarm reached the northern coast of the Gulf of Aqaba, coming from the Sinai desert towards the southeast. Upon reaching the coast, they avoided flying over the water, and instead flew north along the coast. Only after passing the tip of the gulf did they turn east again. Experiments with tethered locusts showed that they avoided flying over a light-reflecting mirror, and when given a choice of a non-polarizing reflecting surface and a surface that reflected linearly polarized light, they preferred to fly over the former. Our results suggest that locusts can detect the polarized reflections of bodies of water and avoid crossing them; at least when flying at low altitudes, they can therefore avoid flying over these dangerous areas.

Animal Migration↗

PubMed2000

Using x-ray reflectivity to determine the structure of surfactant monolayers

Interactions among the multiple degrees of freedom of surfactant molecules cause fascinating richness in the structure of their monolayers. Beyond this scientific motivation for studying surfactant monolayers, the technological use of monolayers for interfacial control and molecular assembly demands a clear understanding of monolayer structure. X-ray and neutron reflectivity have become prime techniques for determining this structure. We present x-ray reflectivity data for a representative surfactant monolayer system and outline an objective procedure for obtaining the maximum amount of structural information possible. Our approach combines tight control of instrumental parameters, dynamically optimized Monte Carlo and simulated annealing to probe the chi(2) hypersurface, and a set of statistical criteria for accepting and rejecting fits. We justify our procedure through tests using simulated data. Results indicate that an ensemble of fits must be performed for each set of reflectivity data in order to survey the chi(2) hypersurface adequately. A single good fit may yield structural parameters which are quite misleading, yet physically plausible. Thus, one must never be satisfied with performing just a single fit. In cases for which multiple, statistically equivalent fits are obtained, the apparent ambiguity is substantially mitigated by averaging the parameters over the ensemble of good fits. We also introduce a method of dealing with cases for which a good fit may be extremely difficult to find. Our analysis procedures can be generalized to other monolayer or multilayer systems and are also applicable to neutron reflectivity.

Journal Article↗

PubMed2002

Wave transport through thin slabs of random media with internal reflection: Ballistic to diffusive transition.

The static and dynamic properties of wave transport through thin slabs of random media in the presence of internal reflection are investigated by performing first-principles calculations. These results are compared with results from time-independent and time-dependent diffusion equations, respectively, where the effects due to internal reflection are incorporated into an average extrapolation length in the boundary conditions. For the static properties, we find an abrupt transition from ballistic to diffusive behavior when sample thickness is about three mean free paths, i.e., L approximately 3l. The diffusion approximation is valid when L>3l, independent of the amount of internal reflection. For the dynamic properties, both the peak arrival time at short times and the diffusion constant at long times of the transmitted pulse indicate that there is a region of anomalous diffusion when 3l L(c). The value of L(c) is about ten times the average extrapolation length, i.e., L approximately 10z(e), where z(e) is a fast increasing function of the amount of internal reflection.

Journal Article↗

PubMed2002

Reflectivity and energy balance in pulsed-laser deposition experiments from mono- and bi-atomic targets.

Reflectivity and complete energy balance from the planar targets of aluminum, copper, nickel, and molybdenum have been measured using a Nd: glass laser (lambda=1060 nm,tau=5 ns) in the low intensity regime of laser plasma interaction as a function of focal spot size. The magnitude of partition of total incident laser energy into different channels is observed to decrease as the focal spot size increases. It is further observed that the magnitude of partition of the incident energy into these channels, in general, decreases as the atomic number increases for any given focal spot size, although, the reflectivity component of the partitioned energy increases with focal spot size for any given element. The reflectivities of copper and tungsten and their alloy were measured separately. The reflectivity from the alloy plasma was reduced by a factor of 6 compared to either element separately. This observation confirms the recent theory that in the multiion plasma the ion acoustic waves are additionally damped due to additional Joule, thermal diffusion, and viscous terms in the modified ion-fluid theory of the ion acoustic waves in a multiion species plasma.

Journal Article↗

PubMed2004

Superluminal pulse reflection and transmission in a slab system doped with dispersive materials.

The reflection and transmission of a pulse through a slab which is doped with two-level or three-level atoms are investigated theoretically. The doped atoms can be passive (absorptive) or active (gain). We find that both the reflected and transmitted pulses can be superluminal simultaneously for the slab doped with absorptive two-level atoms at the slab thickness equal to (2m+1) lambda(0) /4 sqrt[epsilon(b)] (where lambda(0) is the center wavelength of the incident pulse, and epsilon(b) is the background dielectric constant of the slab) or with active three-level atoms at any thickness. By adjusting the thickness or background dielectric constant of the slab, the reflected pulse can be controlled from superluminal to subluminal or vice versa for the slab doped with absorptive two-level or absorptive three-level atoms. The energy percentage in the reflected pulse can also be controlled by changing the thickness of the slab, and the doped atoms.

Journal Article↗

PubMed2001

Specular reflection of very slow metastable neon atoms from a solid surface.

An ultracold narrow atomic beam of metastable neon in the 1s3[(2s)(5)3p:1P0] state is used to study specular reflection of atoms from a solid surface at extremely slow incident velocity. The reflectivity on a silicon (1,0,0) surface and a BK7 glass surface is measured at the normal incident velocity between 1 mm/s and 3 cm/s. The reflectivity above 30% is observed at about 1 mm/s. The observed velocity dependence is explained semiquantitatively by the quantum reflection that is caused by the attractive Casimir-van der Waals potential of the atom-surface interaction.

Journal Article↗

PubMed2001

Resonance absolute quantum reflection at selected energies.

We present one-dimensional local potentials with an absolute reflection at a given energy value which can be above barriers. The corresponding energy dependence of a reflection coefficient exhibits resonance behavior. The inversion technique provides the potentials with specified widths of reflection resonances, their number and positions. The multichannel systems (exact models) with a complete reflection are also given.

Journal Article↗

PubMed2002

Near-threshold reflectivity fluctuations in the independent-convective-hot-spot-model limit of a spatially smoothed laser beam.

In the framework of the independent-hot-spot model, it is shown that the reflectivity resulting from scattering instabilities when a spatially smoothed laser beam interacts with a plasma exhibits large statistical fluctuations near threshold. The importance of the fluctuations is discussed in terms of a confidence interval for the reflectivity, which is more relevant to experimental measurements than the average reflectivity. An analytical model for the fluctuating reflectivity is developed and shown to be in good agreement with numerical simulations. The influence of the transverse size of the interaction region is studied.

Journal Article↗

PubMed2006

Specular Andreev reflection in graphene.

By combining the Dirac equation of relativistic quantum mechanics with the Bogoliubov-de Gennes equation of superconductivity we investigate the electron-hole conversion at a normal-metal-superconductor interface in graphene. We find that the Andreev reflection of Dirac fermions has several unusual features: (1) the electron and hole occupy different valleys of the band structure; (2) at normal incidence the electron-hole conversion happens with unit efficiency in spite of the large mismatch in Fermi wavelengths at the two sides of the interface; and, most fundamentally: (3) away from normal incidence the reflection angle may be the same as the angle of incidence (retroreflection) or it may be inverted (specular reflection). Specular Andreev reflection dominates in weakly doped graphene, when the Fermi wavelength in the normal region is large compared to the superconducting coherence length.

Journal Article↗

PubMed1976

Seasonal changes in reflectance and standing crop biomass in three salt marsh communities.

Reflectance of red (656-705 nm) and infrared (776-826 nm) solar radiation and standing crop biomass were measured in three salt marsh communities at intervals of approximately 2 weeks between February and August 1974. Red reflectance declined at the onset of greening in each community and was correlated with standing crop of green biomass. Infrared reflectance increased substantially in the shrub community but less in the grass and sedge communities. The inverse of red reflectance was found to be a reliable predictor of green biomass in sedge and grass communities, but not in a shrub community.

Journal Article↗

PubMed1992

Polarized dispersion, glide-rule-forbidden reflections and phase determination in barium bromate monohydrate.

Reflections forbidden by a glide-plane rule are observed in diffraction experiments with a crystal of barium bromate monohydrate using linearly polarized synchrotron radiation with wavelength near the bromine K-absorption edge. Their intensities change with azimuth in agreement with equations derived using a tensor model of the anomalous scattering of the bromate ion and are consistent in scale with earlier measurements of that tensor in sodium bromate. The intensity of each forbidden h0l reflection gives the magnitude and phase of the bromine part of the structure factor of the allowed 2h,0,2l reflection. The x and z coordinates of the Br atom determined from such data for 11 reflections are within 0.02 A of those from two crystal structure determinations.

Barium↗

PubMed1999

A 'three-beam' analysis of resonance scattering in reflection high-energy electron diffraction.

Enhanced reflection of fast electrons from a crystal surface and a decrease in the depth of penetration of the primary beam occurs when diffraction conditions are such as to set up a wave travelling just beneath the crystal surface. This is the surface resonance condition for reflection high-energy electron diffraction (RHEED). Quantitative prediction of these effects can be achieved by assuming that only the primary and two diffracted beams are significant. Expressions for the coefficient of reflection and the depth of penetration in terms of a few Fourier coefficients of an effective potential are derived. These coefficients depend sensitively on incident-beam direction and are significantly different from the values for the bulk crystal. In particular, the mean potential experienced by the electrons in the resonance state is increased. It can be estimated using Bethe's perturbation approach. Predictions of the position, height and width of the peak in reflectivity resulting from resonance scattering from the (111) surface of platinum are in reasonable agreement with the values obtained from many-beam computations. The three-beam approach gives insight into resonance scattering using the standard formalism of diffraction theory.

Journal Article↗

PubMed2006

Structure model for the tau(mu) phase in Al-Cr-Si alloys deduced from the lambda phase by the strong-reflections approach.

There are very obvious common features in the electron diffraction patterns of the lambda and tau(mu) phases in the Al-Cr-Si system. The positions of the strong reflections and their intensity distributions are similar for the two structures. The relation of the reciprocal lattices of the lambda and tau(mu) phases is studied. By applying the strong-reflections approach, the structure factors of tau(mu) are deduced from the corresponding structure factors of the known lambda phase. Rules for selecting reflections for the strong-reflections approach are described. Similar to that of lambda, the structure of tau(mu) contains six layers stacked along the c axis in each unit cell. There are 752 atoms in each unit cell, 53 of them are unique. The corresponding composition of the tau(mu) model is Al(3.82 - x)CrSi(x). Simulated electron diffraction patterns from the structure model are in good agreement with the experimental ones. The arrangement of interpenetrated icosahedral clusters in the tau(mu) phase is discussed.

Journal Article↗

PubMed2005

Improving experimental phasing: the role of strongest reflections.

Very strong reflections have a dominant impact on the initial phasing and model-building stages of structure determination. However, experimental phasing (MIR, SAD or MAD) fails on some of the strongest reflections when the heavy-atom contribution to scattering is relatively weak or absent. It is shown that when just a few (approximately 50-100) of these reflections are assigned low-error phases, the entire set of isomorphous replacement phases becomes significantly improved after density modification. This improvement is indicated by higher map correlation coefficients and reduced mean phase errors of the updated data. The problem of phasing the strongest reflections may be solved by the direct measurement of triplet phases in a three-beam diffraction experiment. The analysis shows that merging isomorphous replacement data with a limited number of highly accurate phases from the reference-beam diffraction experiment would significantly improve conventional experimental phasing.

Crystallography, X-Ray↗

PubMed2001

Diffraction anomalous fine structure of forbidden reflection of super-ordered GaInP.

We used DAFS to probe super-ordered domains in InGaP/GaAs epitaxial growth. The sample was lattice matched InGaP epitaxially grown on GaAs with a substrate miscut angle of 6 degrees with respect to the (001) direction. InGaP epi-layer exhibited (111)-type alloy ordering, of alternating InP and GaP like planes and giving rise to a (-5/2,5/2,-5/2) Bragg peak reflection which becomes allowed. Structural data can be extracted, at the same time, for the surroundings of Gallium in the bulk and in the epi-layer from allowed reflections, while the forbidden reflection gives structural details around Gallium in the ordered domains. Difference with the bulk InGaP Fourier transform confirms the symmetry selectivity of chosen reflections for the super-ordered domains.

Journal Article↗

PubMed1992

The relationship of surface reflectance measurements to optical properties of layered biological media.

Reflectance from a turbid biological tissue is discussed for a diffusive light source illuminating the surface of the medium, and is related to the optical property distribution within the medium and to photon propagation through the medium. A three-dimensional photon diffusion model with closed form is developed to describe the photon diffuse intensity in a homogeneous medium. The solution is extended by numerical methods to the medium with layered structure. The concepts of photon flux paths and of reflectance indexes are utilized, together with reflectance data, to extract information about the internal optical properties of a medium. The flux path concept was corroborated by successfully detecting in vivo and ex vivo layered differences in optical properties within the biological medium. These studies suggest that the optical properties of subdermal tissue can be measured from light reflectance and that the effect of the upper skin layers can be eliminated.

Absorption↗

PubMed1993

Temperature dependence of reflectance and transmittance of the artery exposed to air during laser irradiation.

Reflectance, transmittance, and temperature of the arterial wall exposed to air are measured during laser irradiation for different heating dynamics. Temperature dependence of the reflectance and transmittance is then deduced. Our results show a competing effect between temperature and dehydration, resulting in a distinct minimum of transmittance and maximum of reflectance. As a consequence, a direct correlation of reflectance and transmittance with temperature is only possible for a specific dynamics.

Animals↗