Opaque Screen Research Articles

The group velocity of electromagnetic waves diffracted by a small aperture

Diffraction of electromagnetic waves by an aperture in an infinite opaque screen is approached using an integral expression characterized by the spatial Fourier transform at the aperture. The group velocity of the field propagated from the aperture assumes a continuous distribution of values corresponding to the distribution in transverse wavenumber. The gravitational Doppler shift is computed for an aperture that is small compared to a wavelength. It is found that the integral expression can be evaluated analytically, leading to a distribution of Doppler shifts; the minimum Doppler shift is on the order of one part in 10 15. Every additional order of magnitude increase in the Doppler shift results in a decrease in amplitude by two orders of magnitude.

Path–linking interpretation of Kirchhoff diffraction: a summary

The Kirchhoff theory of diffraction of 1882 underpinned the earlier formulations of wave optics of Huygens, Young and Fresnel. Remarkably, it provides an exact explicit solution of the wave equation, with well–defined, though unusual, boundary conditions on the opaque diffracting screen. This prestigious status was not appreciated at the time and, despite the incisive clarification of Kottler in 1923, remains largely unsung to this day. Here I re–present my path–linking interpretation of Kirchhoff diffraction in which the Kirchhoff wave field at any observation position is formally represented as a ‘sum’ of contributions from all possible paths of propagation from the source point, whether or not they pass through the screen. The contributions are shown to be weighted according to the linking number of the path (when closed by an unobstructed return path) with the screen boundary loop(s). Kirchhoff optics is thus shown to be inherently topological in nature.

Surface plasmon propagation in microscale metal stripes

Addressing the fundamental question of miniaturization of light guiding and routing towards nanoscale optics, we study experimentally surface plasmon propagation in silver and gold thin films of finite widths in the micrometer range. Spatially confined excitation of surface plasmons is realized by a prism coupling arrangement involving an opaque aluminum screen for a distinct separation of excitation and propagation (measurement) region. The surface plasmon propagation length as a function of film widths is measured by detecting stray light due to surface plasmon scattering with a conventional optical microscope.

Infant Understanding of the Referential Nature of Looking

To determine whether infants follow the gaze of adults because they understand the referential nature of looking or because they use the adult turn as a predictive cue for the location of interesting events, the gaze-following behavior of 14- and 18-month-olds was examined in the joint visual attention paradigm under varying visual obstruction conditions: (a) when the experimenter's line of sight was obstructed by opaque screens (screen condition), (b) when the experimenter's view was not obstructed (no-screen condition), and (c) when the opaque screens contained a large transparent window (window condition). It was assumed that infants who simply use adult turns as predictive cues would turn equally in all 3 conditions but infants who comprehend the referential nature of looking would turn maximally when the experimenter's vision was not blocked and minimally when her vision was blocked. Eighteen-month-olds responded in accord with the referential position (turning much more in the no-screen and window conditions than in the screen condition). However, 14-month-olds yielded a mixed response pattern (turning less in the screen than the no-screen condition but turning still less in the window condition). The results suggest that, unlike 18-month-olds, 14-month-olds do not understand the intentional nature of looking and are unclear about the requirements for successful looking.

Diffraction as a consequence of Faraday’s law

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Patient Perception of Limb Position during Regional Anesthesia. Isaacson et al. (page 55)

To evaluate the influence of limb position changes on the incidence of incorrectly perceived or phantom sensations during regional anesthesia, Isaacson et al. observed 40 women scheduled to undergo genitourinary procedures with subarachnoid anesthesia. The authors’ objective was to reevaluate the controversy of rigid versus plastic influences on phantom sensations during regional anesthesia and to develop a better understanding of proprioceptive memory–imprinting processes.Preoperative medication was limited to a maximum of 2 mg intravenous midazolam. After administration of subarachnoid block, patients were placed in the supine position and an opaque screen was placed in such a way as to prevent the patients from seeing their legs. Patients scheduled for procedures lasting 1 h or less were assigned to group 1 and were administered 1.5 ml lidocaine, 5%, in 7.5% dextrose; patients scheduled for procedures longer in duration than 1 h were assigned to group 2 and were administered 1.5 ml bupivacaine, 0.75%, in 7.5% dextrose. Then, patients in each group were randomized to one of four time groups: 1, 4, 7, and 10 min. The authors assigned each patient’s dominant leg as the leg to be flexed first (ipsilateral limb). The nondominant (contralateral) leg was completely extended. The patient’s legs were in this position for 1, 4, 7, or 10 min, depending on the time group to which the patient was randomized, and the patient was questioned regarding leg position. At the designated time, leg positions were switched simultaneously. Sensory testing to pin prick was performed 10 min after injection for each group and also was used during repositioning as a distraction to minimize movement cues.The percentage of incorrect responses was analyzed using a logistic regression model with the independent variables of treatment and time. The authors found that inability to perceive a change in limb position was dependent on the point at which the position change was initiated, in relation to the onset characteristics of the local anesthetic. Patients incorrectly identified the position of the flexed-first limb more often than they did the position of the contralateral limb, leading the authors to conclude that proprioceptive memory involves a dynamic neuroplastic imprinting process (rather than a fixed body schema) that is influenced by limb position before onset of regional anesthesia.

Self-reconstruction of an optical vortex

The results of experiments devoted to the diffraction of a light beam with an axial optical vortex from the edge of an opaque screen are presented. The reconstruction of an optical vortex in the transmitted beam is observed under conditions when the screen cuts off the central part of the incident beam.

Inhibition of denitrification by ultraviolet radiation.

It has been shown that UV-A (lambda=320-400 nm) and UV-B (lambda=280-320 nm) inhibit photosynthesis, nitrogen fixation and nitrification. The purpose of this study was to determine the effects, if any, on denitrification in a microbial community inhabiting the intertidal. The community studied is the microbial mat consisting primarily of Lyngbya that inhabits the Pacific marine intertidal, Baja California, Mexico. Rates of denitrification were determined using the acetylene blockage technique. Pseudomonas fluorescens (ATCC #17400) was used as a control organism, and treated similarly to the mat samples. Samples were incubated either beneath a PAR transparent, UV opaque screen (OP3), or a mylar screen to block UV-B, or a UV transparent screen (UVT) for 2 to 3 hours. Sets of samples were also treated with nitrapyrin to inhibit nitrification, or DCMU to inhibit photosynthesis and treated similarly. Denitrification rates were greater in the UV protected samples than in the UV exposed samples the mat samples as well as for the Ps fluorescens cultures. Killed controls exhibited no activity. In the DCMU and nitrapyrin treated samples denitrification rates were the same as in the untreated samples. These data indicate that denitrification is directly inhibited by UV radiation.

The van Cittert-Zernike Theorem

The beam of light emanating from a quasi-monochromatic point source (or a sufficiently distant extended source) is said to be spatially coherent: the reason is that, at any two points on a given cross-section of the beam, the oscillating electromagnetic fields maintain their relative phase at all times. If an opaque screen with two pinholes is placed at such a cross-section, Young's interference fringes will form, and the observed fringe contrast will be 100% (at and around the center of the fringe pattern). This is the sense in which the fields at two points are said to be spatially coherent relative to each other. If the relative phase of the fields at the two points varies randomly with time, the pair of point sources will fail to produce Young's fringes and, therefore, the fields are considered to be incoherent. In practice there is a continuum of possibilities between the aforementioned extremes, and the resulting fringe contrast may fall anywhere between zero and 100%. The fields at the two points are then said to be partially coherent with respect to one another, and the properly defined fringe contrast in Young's experiment is used as the measure of their degree of coherence. Optical systems involving partially coherent illumination are explored in several other chapters of this book; see, for example, “Coherent and incoherent imaging” (Chapter 5), “Michelson's stellar interferometer” (Chapter 35), “Zernike's method of phase contrast” (Chapter 38), and “polarization microscopy” (Chapter 39).

Two Related Forms of Long-Term Habituation in the Crab Chasmagnathus Are Differentially Affected by Scopolamine

BERÓN DE ASTRADA, M. AND H. MALDONADO. Two related forms of long-term habituation in the crab Chasmagnathus are differentially affected by scopolamine. PHARMACOL BIOCHEM BEHAV 63(1) 109–118, 1999.—An opaque screen moving overhead elicits an escape response in the crab Chasmagnathus, which, after a few presentations, habituates for a long period (long-term habituation, LTH). Previous results distinguished two types of LTH: the (context-signal)-LTH yielded by spaced training, determined by an association between context and habituating stimulus, and cycloheximide sensitive; and the (signal)-LTH produced by massed training, context independent, and cycloheximide insensitive. Present experiments were aimed at studying the possible involvement of cholinergic mechanisms in one or both types of LTH, using the muscarinic antagonist, scopolamine (SCP). Results indicate that LTH acquired by spaced training (30 trials separated by 85 s) is blocked in a dose-dependent manner by posttraining SCP. Amnesia is shown with 100 ng SCP/g injected immediately before or after spaced training but not delayed 1-h posttraining. No effect of SCP on LTH acquired by massed training (300 trials separated by 4 s) is detected. Pretraining SCP induces a decrease in the response level at the initial trials of either a spaced or a massed training. It is concluded that the storage of (context-signal)-LTH may be selectively regulated by a muscarinic-cholinergic mechanism. However, the possibility that other cholinergic receptors would be involved in the consolidation of the (signal)-LTH is discussed.

Decreased response or alternative defensive strategies in escape: two different types of long-term memory in the crab Chasmagnathus

An opaque screen moving overhead elicits an escape response in the crab Chasmagnathus that after a few presentations habituates for a long period (long-term habituation). Two types of long-term habituation were previously described: the (context-signal)-long-term habituation yielded by spaced training – context dependent, cycloheximide sensitive and long lasting; and the (signal)-long-term habituation yielded by massed training – context independent, cycloheximide insensitive and shorter lasting. Present research is focused on the defensive strategies crabs display during acquisition of both long-term habituations, using video analysis as the main method of study. Aside from the escape response, Chasmagnathus shows a rigid motionless display, an alternative defensive response we term freezing response. The escape response is predominantly exhibited at night and in summer months, while freezing occurs during day light hours and in winter months. During acquisition of (signal)-long-term habituation, the escape response vanishes without being replaced by freezing. During acquisition of (context-signal)-long-term habituation, the escape response vanishes and is replaced by a strong freezing that finally becomes the only defensive strategy. The former, but not the latter, meets the current concept of habituation.

Formation of a negative image by an opaque screen

An effect that can serve as a basis for reconciling two approaches in the description of diffraction— Fresnel’s and Young’s—is described.

Multi-aperture optical head for parallel scanning near field optical microscopy

This paper proposes the spatial sampling of high spatial frequencies images and their conversion into sequences of diffraction limited images. That method is based on a hybrid optical head which combines a classical diffractive lens and a spatial filter directly placed in the high resolution optical field. That filter is made of a matrix of sub-wavelength apertures etched in a thin opaque screen. The distance L between apertures is large enough to be resolved by the diffractive lens. The elementary aperture size d determines the exact size of object points. If the aperture size d is much smaller than the wavelength used, near field resolutions can be expected. Each image obtained with that optical head is a sub-image (or sample matrix) of the whole high resolution image with a sampling ratio of ( d/ L) 2. The reconstruction of the entire high resolution image is based on a sequence of ( L/ d) 2 diffraction limited sub-images which are recorded during a lateral scanning of the optical head.

Toepler, Mach, and Wood: The visualization of sound

The 19th Century saw great strides in making sound visible. A major pioneer was August Toepler (1836–1912). Toepler studied the so-called singing flame. To make periodic fluctuations visible, he interposed a rotating disc with slits between the flame and the viewing telescope. In so doing, he developed the idea of the stroboscope, which ultimately became a common device. In a second experiment, Toepler focused the image of a spark source; he then covered the image with an opaque screen. If the rays of light passed through a region with a differing index of refraction, the rays would be bent and thus miss the opaque covering. Toepler called this method schlieren beobachtung, or striae observation. The American, R. W. Wood, popularized this second technique. Examples will be shown. Even before Wood, Ernst Mach (1836–1916) applied Toepler’s method to the study of shock waves. Examples will also be shown here.

Context Shift and Protein Synthesis Inhibition Disrupt Long-Term Habituation after Spaced, but Not Massed, Training in the CrabChasmagnathus

An opaque screen moving overhead elicits an escape response in the crabChasmagnathusthat after a few presentations habituates for a long period (long-term habituation, LTH). Previous results suggested that spaced (15 trials separated by 171 s) and massed training (300 trials without rest interval) were correlated with two different memory components of LTH. The present experiments were aimed at further studying the mechanisms subserving these components. Results indicate that LTH acquired by spaced but not by massed training is blocked either by a training-to-testing context shift or by cycloheximide (15–25 μg) pre- or posttraining injection and that LTH after spaced training persists for longer time (5 days) than after massed training (2 days). A model based on these results that distinguishes two LTH-memory components is proposed: a (context-signal) LTH yielded by spaced training, dependent of context, sensitive to cycloheximide (CYX), and long lasting; and a (signal) LTH yielded by massed training, dependent only on the signal invariance, insensitive to CYX, and shorter lasting.

Delayed search for a concealed imprinted object in the domestic chick

Five-day-old chicks were accustomed to follow an imprinted object (a small red ball with which they had been reared) that was moving slowly in a large arena, until it disappeared behind an opaque screen. In experiments, each chick was initially confined in a transparent cage, from where it could see and track the ball while it moved towards, and then beyond, one of two screens. The screens could be either identical or differ in colour and pattern. Either immediately after the disappearance of the ball, or with a certain delay, the chick was released and allowed to search for its imprinted object behind either screen. The results showed that chicks took into account the directional cue provided by the ball movement and its concealment, up to a delay period of about 180 s, independently of the perceptual characteristics of the two screens. If an opaque partition was positioned in front of the transparent cage immediately after the ball had disappeared, so that, throughout the delay, neither the goal-object nor the two screens were visible, chicks were still capable of remembering and choosing the correct screen, though over a much shorter period of about 60 s. The results suggest that, at least in this precocial bird species, very young chicks can maintain some form of representation of the location where a social partner was last seen, and are also capable of continuously updating this representation so as to take into account successive displacements of the goal-object.

Context-us association as a determinant of long-term habituation in the crabChasmagnathus

An opaque screen moving overhead elicits an escape response in the crabChasmagnathus that habituates for a long period after just a few presentations. A series of experiments was performed to determine whether the crab’s long-term habituation (LTH) is mediated by an association between contextual cues and the eliciting stimulus.Chasmagnathus failed to exhibit LTH when a visual cue in the experimental environment was changed between training and testing. In addition, long-term exposure to the context in the absence of the eliciting stimulus impaired LTH, both when the exposure preceded the habituation training (latent inhibition) and when the exposure came after the training (extinction). Long exposure to the context alone prior to training also produced a decrease in responsiveness to the eliciting stimulus, which confirmed previous results. However, both effects of long exposure were only manifested when the crabs spent a period of time between exposure and testing out of the experimental context. The results of this paper are interpreted as supporting the view thatChasmagnathus LTH can be understood largely by Wagner’s associative theory of habituation.

Recognition of form by pre-school children after progressive visual exposure

The purpose of this study was to examine the factors that influence integration of shape information by pre-school children. In a series of four experiments, children saw shapes revealed behind a narrow aperture in an opaque screen and they were then tested for recognition of the shape. In the first experiment, 3-year-old children recognized the shapes presented behind a stationary aperture both with and without a delay of 10 seconds between exposure and test. In Experiments 2A, 2B, and 3 with 2.5-year-old children, it was found that shape recognition was facilitated by a moving aperture as opposed to a moving shape, by up–down as opposed to left–right exposure, and by immediate as opposed to delayed recognition after familiarization with the choice array. It was concluded that recognition of progressively exposed form by 3-year-olds is mediated by integrational and memorial processes that although not fully mature are capable of supporting sophisticated cognitive activities. ©1997 John Wiley & Sons, Ltd.

Recognition of form by pre‐school children after progressive visual exposure

The purpose of this study was to examine the factors that influence integration of shape information by pre-school children. In a series of four experiments, children saw shapes revealed behind a narrow aperture in an opaque screen and they were then tested for recognition of the shape. In the first experiment, 3-year-old children recognized the shapes presented behind a stationary aperture both with and without a delay of 10 seconds between exposure and test. In Experiments 2A, 2B, and 3 with 2.5-year-old children, it was found that shape recognition was facilitated by a moving aperture as opposed to a moving shape, by up–down as opposed to left–right exposure, and by immediate as opposed to delayed recognition after familiarization with the choice array. It was concluded that recognition of progressively exposed form by 3-year-olds is mediated by integrational and memorial processes that although not fully mature are capable of supporting sophisticated cognitive activities. ©1997 John Wiley & Sons, Ltd.

Talbot-like bands for a laser diode below threshold

We report on the problems encountered when replacing a tungsten filament lamp with a laser diode in a set-up for displaying Talbot bands using a diffraction grating. It is shown that the band pattern is rather complex and strong interference signals may exist in situations where Talbot bands are not normally expected to appear. In these situations, the period of the bands increases with the optical path difference (OPD). The visibility of bands as dependence on path imbalance is obtained by suitably obstructing halfway into the arms of a Michelson interferometer using opaque screens.