Showing papers on "Wavelength published in 1971"

Die Bestimmung optischer Konstanten von Metallen durch Anregung von Oberflächenplasmaschwingungen

Abstract: A method is given to determine accurately the optical constants and the thickness of thin films when the real and the imaginary part of the dielectric constants obey the condition ɛ r <- 1, ɛ i ¦ɛ r ¦. The method makes use of the possibility to excite surface plasma waves with the help of the inhomogeneous light wave obtained by total reflexion. The accuracy of the method is pointed out. As an example the optical constants of silver foils in the wavelength interval 4000 to 6000 A are determined.

Surface motion of a semi-elliptical alluvial valley for incident plane SH waves

Abstract: By using the exact analytical solution for the two-dimensional SH-wave propagation in and around an elastic inclusion whose cross section corresponds to one half of an ellipse, we have examined those aspects of the resulting ground motion that are of special interest for earthquake engineering and strong-motion seismology. Computed amplitudes and phases of periodic ground motion display complicated wave-interference phenomena that lead to nearly-standing wave patterns, abrupt changes in the amplification of incident motions along the free surface of the alluvial valley and strong dependence of the overall motions on the incidence angle of SH waves. By comparing the amplification patterns derived from the exact model with the amplifications computed on the basis of an equivalent single-layer model excited by the vertically incident shear waves, we have demonstrated that this approximate representation may lead to meaningful results only if the wavelength of incident waves is longer than the characteristic dimension of the alluvial valley. Although simple, we expect that the model presented in this paper might explain qualitatively the vibrations of some alluvial valleys excited by SH components of strong ground motion.

Measurements of subsonic jet noise and comparison with theory

Abstract: Measurements of the noise field from a 25 mm diameter subsonic air jet are presented. These results are analysed in some detail by determining both the jet velocity dependence and the directivity of the intensity of the radiation in 1/3-octave bands at particular values of the frequency parameter, \[ (fD/V_J)(1-M_c\cos\theta). \] This procedure should ensure that a particular source in a geometrically similar position in the jet is always observed, whatever the jet velocity, diameter and emission angle.These results are compared with the predictions of Lighthil's (1952) theory of convected quadrupoles. It is shown that the theory predicts the variation of the intensity with jet velocity and emission angle provided that the observed frequency is below a certain critical value, which depends on jet diameter and emission angle and is independent of jet velocity. Above this critical frequency, the predicted variations overestimate the measurements and it appears that the convective amplification predicted by the theory is much reduced. The variation of this critical frequency is explained by assuming that substantial interaction occurs between the radiated sound and the jet flow when the wavelength of the sound becomes shorter than the sound path length in the jet flow.

Edge waves and crescentic bars

Abstract: The velocity fields associated with edge waves on a sloping beach are examined as possible causes of sedimentary features which have a regular, rhythmic pattern in the longshore direction. It is shown that standing edge waves provide a satisfactory explanation for the formation of crescentic bars in regions of small tidal range, the bars having a longshore wavelength of one-half that of the edge waves. In the absence of large, incoming surface waves, the edge waves may also form cuspate features on the beach face, with the points of the cusps directly opposite to the horns of the crescentic bars. This situation is commonly observed in nature (Hom-ma and Sonu, 1963). The results suggest that standing edge waves with periods of 30–60 secs and significant amplitudes must occur extensively on real beaches.

Method of Edge Waves in the Physical Theory of Diffraction

Abstract: : The diffraction of plane electromagnetic waves by ideally conducting bodies, the surface of which have discontinuities, is investigated in the report. The linear dimensions of the bodies are assumed to be large in comparison with the wavelength. The method developed takes into account the perturbation of the field in the vicinity of the surface discontinuity and allows one to substantially refine the approximations of geometric and physical optics. Expressions are found for the fringing field in the distant zone. A numerical calculation is performed of the scattering characteristics, and a comparison is made with the results of rigorous theory and with experiments.

A note on the helical movement of micro-organisms.

Abstract: This note seeks to evaluate the self-propulsion of a micro-organism, in a viscous fluid, by sending a helical wave down its flagellated tail. An explanation is provided to resolve the paradoxical phenomenon that a micro-organism can roll about its longitudinal axis without passing bending waves along its tail (Rothschild 1961, 1962; Bishop 1958; Gray 1962). The effort made by tho organism in so doing is not torsion, but bending simultaneously in two mutually perpendicular planes. The mechanical model of the micro-organism adopted for the present study consists of a spherical head of radius ɑ and a long cylindrical tail of cross-sectional radius b, along which a helical wave progresses distally. Under the equilibrium condition at a constant forward speed, both the net force and net torque acting on the organism are required to vanish, yielding two equations for the velocity of propulsion, U, and the induced angular velocity, Ω, of the organism. In order that this type of motion can be realized, it is necessary for the head of the organism to exceed a certain critical size, and some amount of body rotation is inevitable. In fact, there exists 1m optimum head-tail ratio ɑ/bat which the propulsion velocity U reaches a maximum, holding the other physical parameters fixed. The power required for propulsion by means of helical waves is determined, based on which a hydromechanical efficiency η is defined. When the head-tail ratio ɑ/b assumes its optimum value and when b is very small compared with the wavelength λ, η ≃ Ω/ω approximately (Ω being the induced angular velocity of the head, ω the circular frequency of the helical wave). This η reaches a maximum at kh ≃ 0.9 (k being the wavenumber 2π/λ, and h the amplitude of the helical wave). In the neighbourhood of kh = 0.9, the optimum head-tail ratio varies in the range 15 < a/b < 40, the propulsion velocity in 0.08 < U/c < 0.2 (c = ω/k being the wave phase velocity), and the efficiency in 0.14 < η < 0.24, as kb varies over 0.03 < kb < 0.2, a range of practical interest. Furthermore, a comparison between the advantageous features of planar and helical waves, relative to each other, is made in terms of their propulsive velocities and power consumptions.

Nonlinear waves in the pitaevskii--gross equation.

Abstract: Nonlinear waves, solitary and periodic, are studied exactly in the Pitaevskii-Gross equation for the wave function of the condensate of a superfluid. We also study the relationship between these two waves and Bogoliubov's phonon, and the energies associated with these waves. The creation energy of a solitary wave with amplitudeA is proportional toA3/2. Solitary waves show interesting behavior on their collision due to their localized character. The effect of collision on solitary waves can be described by the phase shift. We give a formula of the phase shift on a collision of two solitary waves. We further discuss the decay of an arbitrary initial disturbance into solitary waves.

Diffraction of Electromagnetic Waves by a Conducting Screen Perforated Periodically with Holes

Abstract: The reflection and transmission of a plane wave incident on a thin perfectly conducting screen perforated periodically with circular holes are considered. The spacings between holes may be comparable to or less than the wavelength. The metal screen is assumed to be either backed by a flat dielectric sheet, or sandwiched between two or more dielectric sheets that are used to match the susceptsnce of the screen. The techniques of transmission line analysis and the method of moments are employed to obtain a solution for this general class of boundary-value problems. Numerical results for several cases of interest in both X and Ku bands were obtained and shown to be in good agreement with experimental measurements.

The Spectral Reflectance of Natural Surfaces

Abstract: The amount of solar energy reflected from various soils and types of vegetation has been measured as a function of sun elevation in six different wavelength ranges in the ultraviolet, visible and near-infrared regions of the spectrum. It is shown that there is a significant dependence of reflectance on both wavelength and elevation of the sun for all surfaces for which measurements were made.

Pulse transmission through a dielectric optical waveguide.

Abstract: Waveguide propagation of a pulse-modulated carrier wave is formulated to include distortion due to dispersion in both attentuation and phase velocity. An optimum input gaussian pulse width exists for maximum information carrying capacity. Results are applied to a numerical study of several singlemode glass optical waveguides in which mode and dielectric dispersion may total zero at some wavelength. For our low-loss (20 dB/km) guides in kilometer lengths, information rates of at least 3 x 10(10) bits/sec should be attainable.

Conversion of Electrostatic Plasma Waves into Electromagnetic Waves: Numerical Calculation of the Dispersion Relation for All Wavelengths

Abstract: The dispersion curves have been computed for a wide range of wavelengths from electromagnetic waves to electrostatic waves in a magnetoactive warm plasma with a Maxwellian velocity distribution function. The computation was carried out mainly for the perpendicular propagation mode. The upper hybrid resonance is the connection point of the electrostatic waves and the electromagnetic waves. The electrostatic waves not associated with the upper hybrid resonance are subjected to electron cyclotron damping when the wavelength becomes long. Oblique propagation is allowed for the electrostatic waves in a frequency range from the plasma frequency to the upper hybrid resonance frequency in the long-wavelength region where Landau damping can be neglected and where the electrostatic mode smoothly connects to the electromagnetic X-mode. In a slightly inhomogeneous plasma, the Bernstein-mode electrostatic wave can escape by being converted into the O-mode electromagnetic wave; two reflections take place during this escape process.

On the scattering of aerodynamic noise

Abstract: According to the Lighthill acoustic analogy, the sound induced by a region of turbulence is the same as that due to an equivalent distribution of quadrupole sources within the fluid. It is known that the presence of scattering bodies situated near such multipoles can convert some of their intense near field energy into the form of sound waves whose amplitude is far greater than that of the incident field. Calculations are here presented to determine the extent of this conversion, for hard and soft bodies of various shapes, making use of the reciprocal theorem to recast the problem into one of finding the field, near the obstacle, induced by an incident plane wave. If the obstacle is small compared with a wavelength, then its presence is equivalent to an additional dipole (or source) whose greater efficiency as a sound radiator implies that the familiar intensity law I ∝ U8, for far field intensity I against typical turbulence velocity U for an unbounded flow, is replaced by I ∝ U6 (or I ∝ U4) for a hard (or soft) body. For the situation where the scatterer is large compared with wavelength, the prototype problem of a wedge of exterior angle (p/q)π is shown to yield an intensity law I ∝ U4+2q/p for both hard and soft surfaces. This result is shown to hold for the more general ‘wedge-like’ surfaces, whose dimensions are large scale and whose edges may be smoothed out on a small scale, compared with wavelength. The method used involves the matching of an incompressible flow, on the fine scales typical of the edge geometry, to an outer flow determined by the large scale features of the surface. Favourable comparisons are made with previous results pertaining to the two-dimensional semi-infinite duct and to the half-plate of finite thickness.

Acoustic microscope operating at 100 MHz.

Abstract: THE concept of acoustic microscopy is not new1, but the means for constructing a working device are only now becoming available2,3. As acoustic microscopy relies on mechanical vibration, rather than electromagnetic radiation, it may be possible to visualize features of a specimen which go undetected by conventional methods. The lower propagation velocity of sound compared with that of light, in all known materials, results in a corresponding reduction in wavelength for a given frequency of excitation. The theoretically attainable resolution with acoustic microscopy is limited by the angular aperture and the wavelength in a manner entirely analogous to the optical situation. Present ultrasonic technology limits the upper frequency attainable to about 10 GHz which corresponds to a wavelength of 3000–6000 A in most solids and 1000–2000 A in most liquids.

Internal waves generated by a horizontally moving source

Abstract: The internal waves produced by either a moving body or the collapsing wake behind a moving body in a stratified fluid are calculated asymptotically (at large distances behind the source) on the hypotheses of small disturbances, the Boussinesq approximation, and the slender-body approximation (the transverse dimensions of the body and wake are small compared with the wavelengths of the significant internal waves). Explicit results are given for two, complementary models: (a) a constant-N model, in which the density gradient is constant and (b) a thin-thermocline model, in which the density gradient peaks sharply in a thin layer and is elsewhere negligible. The internal-wave spectrum is continuous in (a) and discrete in (b); however, only the dominant mode is included in the explicit results given for (b). A WKB solution also is given for a thermocline model. This approximation does not give an adequate representation of the dominant mode but does provide estimates of the contributions of the highe...

Apparatus for measuring reflected light under stabilized light source conditions

Abstract: Apparatus is described for simultaneously detecting light reflected from a color reflectance surface at two different wavelength ranges wherein the reflected light detected at one wavelength range is employed to stabilize the light output of the light source directed at such reflectance surface and the reflected light detected at another wavelength range is employed to measure specific characteristics of the reflectance surface.

Liquid crystal optical filter system

Abstract: An optical filter system capable of transmitting light at substantially all wavelengths of incident radiation while simultaneously reflecting radiation at a single wavelength band or plurality of wavelength bands within the incident radiation is described. The system employs liquid crystal films having optically negative properties but which are opposite in intrinsic screw sense.

Enhancement of sporadic E by horizontal transport within the layer

Abstract: The tidal winds in the E region, acting through the wind shear mechanism, produce thin horizontal layers of metallic ions, which descend in time as the phases of the winds change. Gravity waves generated below the E region also have a downward-directed phase velocity and, if this is of the same order of magnitude (but greater than) the descent velocity of the layer, the wave ‘seen’ by the ions in the layer can be almost stationary in time. The interaction of the wave with the layer then produces a very strong horizontal redistribution of the ion density, although hardly affecting the height of the layer. We suggest that this process is responsible for enhancing the ion density at certain locations and producing experimentally observable Es. The ‘sporadic’ nature of the phenomenon then arises from the variable presence of suitable waves, while the horizontal scale of the enhanced layer region is controlled by the horizontal wave length of the gravity wave.

The reflexion of internal/inertial waves from bumpy surfaces

Abstract: When internal and/or inertial waves reflect from a smooth surface which is not plane, there is in general some energy flux which is ‘back-reflected’ in the opposite direction to that of the incident energy flux (in addition to that ‘transmitted’ in the direction of the reflected rays), provided only that the incident wavelength is sufficiently large in comparison with the length scales of the reflecting surface. The reflected wave motion due to an incident plane wave is governed by a Fredholm integral equation whose kernel depends on the form of the reflecting surface. Some specific examples are discussed, and the special case of the ‘linearized boundary’ is considered in detail. For an incoming plane wave incident on a sinusoidally varying surface of sufficiently small amplitude, in addition to the main reflected wave two new waves are generated whose wave-numbers are the sum and difference respectively of those of the surface perturbations and the incident wave. If the incident wave-number is the smaller, the difference wave is back-reflected.

Third-harmonic resonance in the interaction of capillary and gravity waves

Abstract: The method of multiple scales is used to determine the temporal and spatial variation of the amplitudes and phases of capillary-gravity waves in a deep liquid at or near the third-harmonic resonant wave-number. This case corresponds to a wavelength of 2·99 cm in deep water. The temporal variation shows that the motion is always bounded, and the general motion is an aperiodic travelling wave. The analysis shows that pure amplitude-modulated waves are not possible in this case contrary to the second-harmonic resonant case. Moreover, pure phase-modulated waves are periodic even near resonance because the non-linearity adjusts the phases to yield perfect resonance. These periodic waves are found to be unstable, in the sense that any disturbance would change them into aperiodic waves.

Measurements of the Attenuation of Sound by a Warm Air Fog

Abstract: Measurements of sound attenuation by a warm air fog are presented for dimensionless frequencies, ωτl/Cm (where τt is the thermal relaxation time for the droplet, ω the circular acoustic frequency, and Cm the liquid mass fraction), near unity where the effects of mass transfer are dominant The tests are made in a Wilson cloud chamber by measuring the rate of decay of the fundamental mode of acoustic oscillation which is excited during the operation of the chamber Measurements of pressure and volume are made continuously during the expansion Droplet size and concentration of the monodisperse fog are determined from time-resolved measurements of the optical transmission at two wavelengths of light The range of ωτl/Cm from 05– 16 is scanned by varying droplet size and concentration through the controlled addition of condensation nuclei This range includes the Napier frequency (frequency for which attenuation per unit wavelength is maximum), and the resulting data indicate that a maximum dimensi

A study of Fetch‐limited wave spectra with an airborne laser

Abstract: An airborne laser wave-profiler was used to estimate fetch-limited, one-dimensional wave number spectra by flying seaward of Cape Henlopen, Delaware, to a distance of 95 nautical miles, following passage of a cold front associated with a mean horizontal wind speed of 14 meters sec−1. The equilibrium range constant was estimated at 4.2×10−3, and it is shown that the constant may be overestimated or underestimated depending on assumed angular spreading of the waves. ‘Overshoot’ observed in these one-dimensional wave number spectra is associated with angular spreading. Linear and exponential growth parameters estimated from fetch-limited wave spectra are in reasonable agreement with other field investigations. Observations of exponential wave growth generally support the Miles-Phillips predictions in the range 12≤C/U*≤21 where C is the phase speed of the wave and U* is the friction velocity.

Time-harmonic waves traveling obliquely in a periodically laminated medium.

Abstract: : The dispersion relation is presented for time-harmonic waves propagating in an arbitrary direction in a periodically laminated medium. The analysis is based on two-dimensional equations of elasticity. Limiting phase velocities are presented for infinite wavelength for any angle of propagation in the form of a fourth-order determinant that illustrates the influence of an arbitrary angle. For the cases when the propagation is along or across the layers, the determinant reduces to two determinants of second order that yield the limiting phase velocities directly. Numerical results are presented to indicate the dependence of dispersion upon the angle of propagation. Also, a comparison with an approximate continuum theory is included; agreement is satisfactory for those angles where the dispersion is the strongest. (Author)

Second Moment of a Wave Propagating in a Random Medium

Abstract: The Bethe–Salpeter equation for the second moment of a wave propagating in a random media is solved in the ladder approximation. The solution obtained applies to propagation problems in which the wavelength is much smaller than the scale of the inhomogeneities, and the product of the rms index fluctuation, the wavenumber, and a characteristic scale length of the inhomogeneities is also much smaller than unity. The field is assumed given on an initial plane and to have appreciable amplitude over a domain many wavelengths in extent in this plane. It is not required, however, that this domain be larger than the scale of the inhomogeneities. Quantitative results are given for a gaussian beam. We show that the ensemble-averaged distribution of irradiance remains gaussian and give the e-folding radius of this distribution. The two-point correlation function of the field in such a beam is shown to depend only on the distance between the points, not on their relative position within the beam. The coherence properties of the beam are expressed in terms of a coherence length. We also consider the ensemble-averaged interference pattern of the field from multiple apertures and verify that well-defined interference fringes occur only when the apertures are separated by a distance smaller than the coherence length of the individual beams. The solution of the Bethe–Salpeter equation obtained here satisfies an equation derived by Beran. We show that the local-independence assumption used by Beran is valid for sufficiently weak inhomogeneities.

Calculation of Second‐Harmonic Generation in a Piston Beam

Abstract: We discuss the nonlinear generation of a second‐harmonic component in the first‐harmonic beam from a circular plane piston, radiating into a fluid. First‐order perturbation theory is used; higher harmonics, absorption, and depletion of each harmonic are ignored, although a partial correction for the last two effects is applied later. The ratio of piston diameter to first‐harmonic acoustic wavelength is assumed to be about 30 or greater. In particular, the axial behavior of the second harmonic, which has marked structure, is found to agree well with experimental results in water. Also, the average value of the second‐harmonic field is calculated over a circle coaxial with the source, parallel to it, and of the same size. This average differs considerably from the result of an oversimplification that treats the first‐harmonic beam as cylindrically collimated plane waves. When compared with measurements in water, the average agrees well at a source pressure of 3 atm but progressively less well at 5 and at 6 ...

Strong infrared‐light scattering from coherent spin waves in yttrium iron garnet

Abstract: We report the first direct observation of strong Bragg‐scattered infrared light (1150 nm wavelength) from coherent spin waves (1.5 GHz). The maximum scattered‐light intensity is found to be at least five times stronger than that of longitudinal elastic waves of comparable power. The experiments were performed with an axially magnetized, single‐crystal YIG bar, with the axis oriented in a {100} plane at an angle of 22.5° from a 〈100〉 axis. The coherent spin waves were generated through efficient space‐gradient conversion from longitudinal elastic waves; the latter were excited from a sputtered ZnO transducer. The scattered‐light intensity is found to be dependent on the incident‐light polarization and the sign of frequency shift (Stokes or anti‐Stokes lines) and independent of the direction of the applied magnetic field. Previous theoretical calculations on light scattering from coherent spin waves are critically reviewed and revised. The new result appears to explain adequately not only the present experiments but also observations that previous authors have reported as ``anomalous''.

High speed optical wavelength detection system

Abstract: Light whose wavelength is to be measured is applied to a device utilizing a light processing means which serves to transmit light to a detector array in a manner determined by its wavelength. The coded output of the detector array can thus immediately indicate the wavelength of the incoming light. I prefer to utilize a wedge interference filter to produce a line of light extending across the width of the filter at a position along its length, which position is a function of the wavelength. The position of the line of light is encoded by a binary code mask to digitally indicate the light wavelength. Indicator lamps are activated in accordance with the digital indication and are rendered responsive to the light from the code mask in response to a high sensitivity detector. The system broadly includes apparatus for measuring a light signal in which a plurality of signal transmission paths whose transfer functions are different functions of light wavelength effect transmission of the light signal to a unique combination of the output channels responsive to a specific input wavelength.

Wave-flow theory for a thin viscous liquid layer

Abstract: On the basis of a simplified system of equations we study the process of development and stability of wave flows in a thin layer of a viscous liquid. Any unstable disturbance of the laminar flow grows and leads to the establishment of the wave regime. The time to establish the flow changes little for large flow rates, but increases sharply with reduction of the flow rate. Given the same amplitudes of the initial disturbances, the optimum regimes which provide the greatest flow rate in a layer of given average thickness develop more rapidly than the other regimes. All the wave regimes are unstable to disturbances in the form of traveling waves. With moderate flow rates, the optimum regimes will be most stable to near-by disturbances. Strictly periodic wave flows in a thin layer of a viscous liquid under the influence of the gravity force were calculated in [1], Various flow wave regimes which differ in wavelength can theoretically be established for a given liquid flow rate. In particular, there is a wavelength for which the flowing layer exhibits minimum average thickness (and maximum flow rate for a given average thickness). These optimum regimes correspond closely to the experimental data [2]; however, with regard to calculation technique these regimes are no different from the others. In the following we make a comparison of the wave regimes on the basis of the nature of their development and stability.

Polarization properties of a grating spectrograph.

Abstract: The 13.7-m Czerny-Turner spectrograph at the McMath solar telescope is evaluated with regard to polarization properties. Let I‖ = the transmissivity of the spectrograph for incident light linear polarized parallel to the entrance slit and I⊥ = transmissivity for light polarized perpendicular to the entrance slit. The ratio I‖/I⊥ was measured photoelectrically as a function of wavelength in six diffraction orders. Values of this ratio vary from 0.3 to 20. It is shown that spectrograph transmission as a function of polarization may cause large photometric and radiometric errors. This fact is not unique to this particular instrument. Two mechanisms appear to contribute to the polarizance of the grating. These are: (1) the Rayleigh or Wood’s anomalies in which polarizance maxima and minima occur and (2) a vector wave interaction introduced because groove dimension is nearly the same as the wavelength. For λ > groove depth it is found that the wavelength of a peak polarizance (λp) is given by λp = 0.7d cosθ, where d is the ruling separation and θ is the grating angle. Photoelectric scans of the solar spectrum were made across several of the Wood’s anomalies to obtain their profiles.