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Gespiegelt in besondern Düften …: Oasen, Seeungeheuer und weitere Spielereien der Fata Morgana
(1998)
„Wer die Natur liebt, der braucht das Beobachten ihrer Erscheinungen wie die Luft zum Atmen.” — So formulierte Marcel Minnaert das Bedürfnis der Naturbeobachtung im Vorwort seines berühmten Buchs „Licht und Farbe in der Natur” [1]. Bekannte Beispiele für Naturerscheinungen in der Atmosphäre sind Luftspiegelungen, Regenbögen und Halos. Diese optischen Phänomene bieten nicht nur faszinierende Anblicke, sondern beinhalten auch genügend Aspekte für eine interessante Physik (z. B. [2]). — Der vorliegende Artikel geht von grundlegenden physikalischen Betrachtungen aus und zeigt, daß sich alle Phänomene atmosphärischer Optik übersichtlich klassifizieren lassen. Luftspiegelungen werden exemplarisch im Detail behandelt. Viele nützliche Informationen findet man in Standardwerken [1, 3–5]; den aktuellen Stand der Forschung in Konferenzberichten, z. B. [6].
Twice in a blue moon
(2009)
The surface to volume ratio in thermal physics: from cheese cube physics to animal metabolism
(2008)
Measurements of illuminance during a solar eclipse are presented. The data are compared to theoretical predictions, based on a geometrical model for obscuration. The model assumes a straight and uniform motion of the sun and moon as well as a spherical shape of both, i.e. it neglects any effects of limb darkening. Furthermore, the sun's disk is assumed to have homogeneous luminosity, i.e. any luminosity variations due to sun spots are neglected. Input parameters are the duration of the eclipse, the duration of totality, the impact parameter, i.e. the distance between the two trajectories of sun and moon, and the sizes of sun and moon. The model applies to all types of eclipses, partial, annular and total.
A geometric model for the optical air mass of the atmosphere is developed. Using the model, simple formulae are derived for the optical thickness of light passing through (1) a molecular atmosphere, (2) an atmosphere with uniformly distributed tropospheric aerosols and (3) atmospheres with elevated aerosol layers. The formulae are used to model the spectra and perceived colours of the Sun and Moon.
Porträts von Naturwissenschaftlern und sachliche Darstellungen aus der Physik finden sich in zahlreichen Bereichen des täglichen Lebens, insbesondere auch auf vielen Banknoten. Die Auswahl -- vorgegeben durch die politisch inspirierte Motivwahl der Notenbanken -- stellt eine interessante Reise durch die Wissenschaftsgeschichte dar.
Strahlungsgesetz und Augenempfindlichkeit: Bemerkungen zu einer klassischen Fehlinterpretation
(2001)
A procedure to experimentally simulate pollen coronas is discussed. Observed coronas are due to pine and birch pollen having different geometries. Using computer simulations, two-dimensional projections of a large number of pollenlike objects with adjustable shapes, with or without preferential orientation and statistical or regular spatial distribution, are generated. The photograph of the printout allows samples with typical sizes between 20 and 200 µm. Their diffraction patterns can closely resemble the ones observed in nature and predicted by theory.
© 2005 Optical Society of America
Thermography of Microsystems
(2004)
Some laboratory demonstrations on atmospheric optics are presented. The focus is on dispersion effects in mirages, lateral mirages, and inferior mirages produced with small hot plates. We also show a demonstration of the upper-tangent-arc halo, produced with a hexagonal prism, rotating about two axes.
© 2003 Optical Society of America
Old and new laboratory experiments on atmospheric optics with a focus on mirages, rainbows, and halos are presented. Some qualitative demonstrations serve primarily didactical purposes, e.g., by proving the existence of curved light rays in media with a gradient of the index of refraction, by directly visualizing the minimum-deviation curve for rainbow paths in water droplets, or by helping to elucidate the ray classes in hexagons that contribute to a specific halo. In addition, quantitative experiments allow a direct comparison of angular positions and intensities with analytical computations or Monte Carlo simulations of light scattering from small water droplets or ice hexagons. In particular, the latter can help us to understand complex halo phenomena.
© 1998 Optical Society of America
Small sodium and silver particles were generated on dielectric substrates like LiF, quartz and sapphire under ultrahigh vacuum conditions. The optical transmission spectra of the clusters were measured as a function of cluster size and shape, for low and high substrate temperatures as well as for s- and p- polarization of the incident light. Excitation of dipolar surface plasmon oscillations in the directions normal and parallel to the substrate surface could be identified. Furthermore, optical spectra for Na and Ag clusters were calculated with the classical Mie theory. The measured spectra vary strongly if the experimental conditions are changed and can be exploited, for example, to characterize the particles with regard to their size and shape. In particular, the axial ratio of the spheroidal clusters could be determined. Its value is considerably different for the two investigated metals and depends on the substrate material. Furthermore, the temperature of the substrate has a pronounced influence on the shape of the particles. At low temperature of T=100 K two-dimensional island growth is predominant. The particles extend only little in the direction perpendicular to the surface and coalesce readily at small coverage of metal atoms. In contrast, the clusters are truly three-dimensional at T=300 K. At this stage, sodium particles still exhibit a rather small axial ratio whereas silver clusters appear almost spherical. Thus, measurements of the optical spectra permit direct in situ monitoring of cluster growth during the nucleation of adsorbed atoms and of temperature induced shape variations. In addition to investigations of the shape of the particles, the quadrupolar surface plasmon mode was observed for Ag clusters.
Width of cluster plasmon resonances: Bulk dielectric functions and chemical interface damping
(1993)
The damping of collective electron resonances in clusters which develop into plasmon polaritons at larger sizes is investigated for free, supported, and embedded neutral metal clusters. Embedding of free 2 nm Ag clusters of 2-nm diameter into a SiO2 matrix leads to an increase of the width of the resonances by more than a factor of 3. The optical spectra are compared with the Mie theory using size-effect-modified dielectric functions of the solid state. The results corroborate the assumption that the widths of the resonances strongly depend on chemical interface effects. The results are briefly discussed with regard to limited-mean-free-path and quantum-size-effect theories and a recent approach by Persson. It is demonstrated that the widths of the spectra of supported and embedded clusters have to be interpreted with care since true intrinsic size effects of the clusters appear to be less effective than previously believed and can be obscured by the chemical interface damping.
© 1993 The American Physical Society
Laser-induced desorption of metal atoms from the surface of small metal particles has been investigated as a function of the shape of the particles and the polarization of the incident laser light. The particles were supported on LiF, quartz or sapphire substrates. In a first set of experiments, the shape of the particles was determined by recording optical transmission spectra with s- and p-polarized light incident under an angle of typically 40° with respect to the surface normal. The metal particles turn out to be oblate, the ratio of the axes perpendicular and parallel to the substrate surface being on the order of 0.5. This ratio decreases with increasing particle size. Also, the particles change shape if the temperature is raised. In further experiments, s- and p-polarized light has been used to stimulate desorption of atoms via surface plasmon excitation. It is found that the desorption rate markedly depends on the polarization of the light. This is explained by excitation of the collective electron oscillation along different axes of the non-spherical particles.
Results on laser-induced desorption of metal atoms from small metal particles are presented. Experiments have been performed on sodium, potassium, and silver particles supported on a LiF(100) single-crystal surface under ultrahigh vacuum conditions. Measurements include the determination of the desorption rate as a function of laser wavelength, laser intensity, average particle size, and substrate temperature, the determination of the kinetic energy of the desorbed atoms, the investigation of the optical spectra of the supported metal particles, and the study of the influence of adsorbate molecules on the desorption rate. Furthermore, theoretical extinction and absorption spectra of the metal particles have been calculated with the classical electrodynamical Mie theory as a function of average particle size and excitation wavelength. Also, the radial electric field at the particle surface was computed. The results of the experiments and theoretical calculations are combined to give a consistent picture of the mechanism of metal-atom desorption by electronic excitation with laser light. A realistic surface potential from which the atoms escape and nonlocal optical effects are taken into account. The latter introduce additional absorption channels by the formation of electron-hole pairs in the surface layer of the particle which relax into antibonding states before desorption occurs. Finally, the mechanism is discussed in the light of similar phenomena observed for thin metal films. Possibilities for future work are outlined.
© 1993 The American Physical Society
Studies of the evolution of the optical properties of metal clusters as a function of size have gained considerable attention in the last few years. One theoretical approach starts from large metal clusters, which can be described by classical electrodynamics, provided the dielectric functions of the clusters are known. the resulting resonant features in the absorption spectra are commonly called surface plasmons and are collective excitations of the electron system.The present paper discusses the electrodynamic (Mie) theory for large clusters of different metals, also considering the range of validity of this approach towards smaller cluster sizes. More details can be found in an extended review on this topic which is to be published soon [1].
Irradiance during total lunar eclipses is simulated using a pinhole model. The Moon is illuminated by direct sunlight that is refracted into the Earth’s shadow as it passes through the atmosphere at the terminator but is depleted by scattering by molecules, extinction by aerosol particles, absorption by ozone, and obstruction by clouds and elevated land. On a spherical, sea-level Earth, and a cloudless, molecular atmosphere with no ozone, the eclipsed Moon appears red and calculated irradiance at the center of the umbra is reduced by a factor of about 2400 from direct moonlight. Selective absorption mainly of light around 600 nm by stratospheric ozone turns the periphery of the umbra pale blue. Typical distributions of aerosol particles, ozone, mountains, and clouds around the terminator reduce irradiance by an additional factor of the order of 100.
© 2008 Optical Society of America
Irradiance and color during the total lunar eclipses of 2007 and 2008 are simulated using a ray tracing model that includes refraction, scattering by molecules, and observed or climatological distributions of aerosols, ozone, clouds, and topography around the terminator. Central portions of the umbra appear deep red for almost all eclipses due to preferential removal of short wavelengths in the spectrum of sunlight by scattering in the lower troposphere. The fringe of the umbra appears turquoise or blue due to selective removal of wavelengths around 600 nm by the Chappuis absorption bands of ozone in the stratosphere. Asymmetric distributions of clouds and aerosols, particularly for the 2008 eclipse, produce minimum calculated irradiance up to 17 arc min from the umbra center, while high ozone content over the arctic makes the northern edge of the umbra deepest blue.
© 2008 Optical Society of America
The Moon’s time-dependent luminance was determined during the 9 February 1990 and 3 March 2007 total lunar eclipses by using calibrated, industry standard photometers. After the results were corrected to unit air mass and to standard distances for both Moon and Sun, an absolute calibration was accomplished by using the Sun’s known luminance and a pre-eclipse lunar albedo of approximately 13.5%. The measured minimum level of brightness in the total phase of both eclipses was relatively high, namely −3.32 mvis and −1.7 mvis , which hints at the absence of pronounced stratospheric aerosol. The light curves were modeled in such a way as to let the Moon move through an artificial Earth shadow composed of a multitude of disk and ring zones, containing a relative luminance data set from an atmospheric radiative transfer calculation.
© 2008 Optical Society of America
We present simple radiative transfer models for the radiance and color of atmospheric optical phenomena. Skylight, halos, and rainbows are treated as singly scattered sunlight that is depleted by scattering as it passes through a plane-parallel atmosphere and a vertical rain shaft or a geometrically thin cloud layer. Skylight in a molecular atmosphere grades from deep blue at the zenith to pale blue near the horizon whenever the solar zenith angle sun ≤ 80°. Skylight near the horizon is orange resulting from wavelength-dependent scattering by air molecules and aerosol particles through a long oblique path through the atmosphere when the sun is low in the sky (sun ≥ 85°). Halos (and coronas) seen through clouds facing the sun are brightest for cloud optical depth τcld ≈ cos(sun), and fade to obscurity for τcld ≥ 5. Rainbows (and glories), seen by light that is backscattered from clouds, also appear most dramatic when 0.2 ≤ τcld ≤ 1, but remain visible even in the thickest clouds.
Light scattering from small particles changes if the particles are absorbing. Whereas the effect is small for coronas and Bishop's ring, glories show pronounced attenuation with increasing absorption. Results indicate suitable wavelength regions for studies of glory scattering from cloud tops. The behavior of core-shell particles could have applications for studying the atmosphere of Venus; in addition it provides more insight into the simple ray-path model of the glory.
© 2005 Optical Society of America
Model simulations of laboratory-generated and natural crepuscular rays are presented. Rays are created in the laboratory with parallel light beams that pass through artificial fogs and milk–water solutions. Light scattered by 90° in a dilute mixture of whole milk first increases in intensity with distance from the source to a maximum as a result of multiple scattering by mainly small angles before decreasing exponentially due to extinction as distance continues to increase. Crepuscular rays are simulated for three cloud configurations. In case 1, the Sun at the zenith is blocked by a cloud with an overhanging anvil. The rays appear white against blue sky and are brightest when atmospheric turbidity, β≈11 . Shading by the anvil separates maximum brightness from apparent cloud edge. In case 2, a ray passes through a rectangular gap in a cloud layer. The ray is faint blue in a molecular atmosphere but turns pale yellow as β and solar zenith angle, ϕsun , increase. At ϕsun=60° it appears most striking when the cloud is optically thick, β≈5 , and the beam width Δx≈1000 m . In these cases, increasing aerosol radius, raer , to about 1000 nm brightens, narrows, and shortens rays. In case 3, the twilight Sun is shaded by a towering cloud or mountain. The shaded rays are deeper blue than the sunlit sky because the light originates higher in the atmosphere, where short waves have suffered less depletion from scattering. The long optical path taken by sunlight at twilight makes color and lighting contrasts of the rays greatest when the air is quite clean, i.e., for β−1≪1 . In all cases, the brightest rays occur when sunlight passes through an optical thickness of atmosphere, τ≈O(1) .
© 2011 Optical Society of America
Rainbows, water droplets, and seeing—slow motion analysis of experiments in atmospheric optics
(2011)
Many physics processes underlying phenomena in atmospheric optics happen on a rather short time scale such that neither the human eye nor video cameras are able to analyze the details. We report applications of high-speed imaging of laboratory experiments in atmospheric optics with subsequent slow motion analysis. The potential to study respective transient effects is investigated in general and for a few phenomena in detail, in particular for rainbow scattering due to single oscillating droplets during free fall, and for light propagation effects through atmospheric paths with turbulences, leading, e.g., to scintillation of stars or shimmering of mirage images.
© 2011 Optical Society of America
Solar load and reflection effects and respective time constants in outdoor building inspections
(2009)
Beleuchtet man die Oberfläche eines Festkörpers mit Laserlicht, können darauf adsorbierte Atome oder Moleküle abgelöst werden. Solche Desorptionsprozesse sind in jüngster Zeit in zahlreichen Experimenten untersucht worden, wobei sich das Interesse sowohl auf die Erforschung der zugrundeliegenden Mechanismen als auch auf mögliche Anwendungen dieser Reaktionen, zum Beispiel zur gezielten Modifizierung von Oberflächen, konzentriert. Besonders interessant ist das laserinduzierte Aufbrechen von Bindungen zur Oberfläche dann, wenn es nicht einfach als thermischer Prozeß durch eine Temperaturerhöhung hervorgerufen wird, sondern direkt auf einer elektronischen Anregung beruht. Solche nichtthermischen Prozesse werden selbst für Metalle beobachtet, obwohl die Kopplung der elektronischen Anregung an das Substrat hier besonders schnell abläuft. Der zugrundeliegende Mechanismus beruht auf dem Zusammenspiel einer kollektiven Elektronenoszillation und einer lokalisierten Einzelelektronenanregung, wobei auch nichtlokale optische Effekte in der Metalloberfläche eine wichtige Rolle spielen.
Gasdetektion mit Infrarotkameras ist eine relativ neue Anwendung. Aufgrund der industriellen Bedeutung einiger Gase wurden sensitive Kameras mit Schmalbandfiltern entwickelt. Heute gibt es kommerzielle Systeme für die Detektion flüchtiger Kohlenwasserstoffe, Schwefelhexafluorid und Kohlenmonoxid. In dieser Arbeit werden Laborergebnisse der Detektion von CO2 vorgestellt und potenzielle Anwendungen diskutiert.
Über die Farben der Sonne und des Himmels: Rayleigh- und Miestreuung in Demonstrationsexperimenten
(2002)
Double pane windows are common objects which can enrich physics teaching at undergraduate level at least in five different fields. First, having sealed inner spaces filled with gas, one can discuss gas law problems upon changes of pressure and/or temperature. Second, when discussing temperature differences between inside and outside, one needs to take into account the associated heat transfer mechanisms which define the pane temperatures, enclosing the gas. Third, using elastic properties of the glass, one may treat deformations of the window panes upon those changes or additional manually applied external pressure. Fourth, the reflective properties of glass combined with the pane deformations result in concave or convex mirrors, which when illuminated by the Sun, may lead to focal points on projection areas such as facing houses. Fifth, such areas receive an increased irradiance which leads to associated thermal effects. Starting from the most obvious daily life phenomenon, the fascinating caustics of reflected sunlight on streets or walls, all of these double pane window phenomena are investigated experimentally as well as theoretically.
Wohl jeder, der schon einmal mit Physik zu tun hatte, kennt die meist grünlichen Leuchtspuren von Oszillographen. Sie ermöglichen es, zeitabhängige Signale zu visualisieren und quantitativ auszumessen, insbesondere auch dann, wenn die Phänomene sehr schnell ablaufen. Die Reaktionsdynamik der leuchtenden Phosphore auf den Bildschirmen wird mit Hochgeschwindigkeits-Videos deutlich sichtbar.
Wenn es knallt und kracht
(2014)
An die Knallgasprobe erinnert sich wohl noch jeder aus dem Chemieunterricht. Die Zeitskala, auf der Wasserstoff und Sauerstoff reagieren hängt davon ab, ob die beiden Reaktionsgase zu Beginn getrennt oder gemischt vorliegen. Dies zeigt sich deutlich in Hochgeschwindigkeits-Aufnahmen von Experimenten, bei denen Luftballons entweder mit reinem Wasserstoff oder aber mit Knallgas gefüllt werden.
Konkav gewölbte Glasfassaden von Großbauten können bei ungünstiger Orientierung zur Sonne zum Brennspiegel werden. Unfreiwillig berühmt gewordene Beispiele sind das Vdara-Hotel in Las Vegas und das “Walkie-Talkie”-Hochhaus in London. Dieser Effekt tritt auch nur an bestimmten Tagen bei passendem Sonnenstand auf. Da die Fassaden keine perfekten Parabolspiegel sind, produzieren sie keinen scharfen Brennpunkt, sondern aufgeweitete Brennflecke, sogenannte Kaustiken. Trotzdem können dort hohe Bestrahlungsstärken auftreten. Simulationen und Experimente mit Modellen bestätigen dies.
Regenbögen, Halos, Koronen, Himmelsfarben – es gibt eine Vielzahl farbenprächtiger Phänomene in der Natur, verursacht durch Reflexion, Brechung oder allgemein gesprochen durch Streuung des Lichts an Wassertropfen, Eiskristallen oder Aerosolen. Das farbige Glitzern einer Schneeschicht scheint zunächst anderer Natur zu sein, lässt sich aber auf analoge Weise erklären.
Ein einfacher physikalischer Zaubertrick besteht darin, eine Kette über den Handrücken zu legen und danach von unten einen Ring über die Kette zu schieben. Der Ring kann entweder von der zweiten Hand oder der die Kette haltenden Hand gehalten werden. Er wird sodann losgelassen und soll (mit einem Knoten) in der Kette hängen bleiben.
The content of a student lab course is described which deals with characterization of the most important parameters governing the performance of infrared cameras. In detail, the parameters describing the temperature resolution, spatial resolution and time resolution of commercial IR cameras are analysed using simple lab experiments. Understanding the respective performance of the system is the prerequisite for applying infrared thermal imaging to quantitative measurements in physics and technology.
The physics behind the sometimes strange effects and 'unnatural' appearance of near-infrared (NIR) photographs is discussed in terms of reflection, absorption and transmission of NIR radiation with the respective objects. Besides discussing how NIR cameras work, several visible and NIR photograph pairs are presented, which include vegetation, natural water, clouds, the sky, and humans. In addition, some physics-oriented experimental NIR images are presented which clearly demonstrate some of the basic physics behind some of these awesome sights.
nfrared spectrometry is one of the most important tools in the field of spectroscopic analysis. This is due to the high information content of spectra in the so-called spectroscopic fingerprint region, which enables measurement not only of gases, but also of liquids and solids. Today, infrared spectroscopy is almost completely dominated by Fourier transform infrared (FTIR) spectroscopy. FTIR spectroscopy is able to detect minute quantities in the ppm and ppb ranges, and the respective analyses are now standard tools in science as well as industry. Therefore FTIR spectroscopy should be taught within the standard curriculum at university to physicists and engineers. Here we present respective undergraduate laboratory experiments designed for students at the end of their third year. Experiments deal first with understanding the spectrometer and second with recording and analysing spectra. On the one hand, transmission spectra of gases are treated which relate to environmental analytics (being probably the most prominent and well-known examples), and on the other hand, the focus is on the transmission and reflection spectra of solids. In particular, silicon wafers are studied—as is regularly done in the microelectronics industry—in order to characterize their thickness, oxygen content and phonon modes.
Infrared
(2013)
A cone of light appears in a tank of water when a laser pointer shines through the water onto a white piece of paper upon which the tank is sitting. We describe how students can understand the origins of this cone by constructing multiple explanations, then proposing and designing experiments to test their explanations. This process is the foundation of the Investigative Science Learning Environment (ISLE) framework, designed to engage students in the reasoning activities similar to those that physicists use to construct and apply new knowledge. We describe typical student ideas and provide a list of equipment and suggestions for facilitating student exploration relating to optics. We also explain the formal physics behind the phenomena that are involved in the experiment. Finally, we suggest how the ISLE framework can be used to help instructors find problems and experiments that engage students in devising and testing multiple explanations.
Rohe Eier oder wassergefüllte Christbaumkugeln platzen beim Eindringen des Projektils einer Luftpistole. Dieser Vorgang lässt sich einfach durch die sehr geringe Kompressibilität des Wassers und eine damit verbundene starke Druckerhöhung erklären (Physik in unserer Zeit 2013, 44(5), 251). Eine genaue Analyse aller Vorgänge sollte aber auch Fragen beantworten wie: Spielen vorhandene Luftblasen in der Flüssigkeit eine Rolle? Wie lange dauert es bis zum Aufplatzen, und muss die wassergefüllte Kugel eigentlich fest verschlossen sein?
Abstract
The brilliant visible colors of various hot springs and pools in Yellowstone National Park are explained with a combination of scattering from the water and from microbial mats that coat the bottoms of these thermal features. A simple 1D radiative transfer model was used to simulate the colors recorded in visible photographs and the spectrum of light making up these colors. The model includes attenuation in water by absorption and molecular scattering as well as reflection characteristics of the microbial mats and surface reflection of the water. Pool geometries are simulated as simple rough cones scaled to have depths and widths that match published data. Thermal images are also used to record the spatial distribution of water skin temperature. The measurements and simulations confirm that colors observed from shallow-water features arise primarily from the spectral properties of the microbial mat, which is related to the water temperature, while colors observed from deeper water arise primarily from the wavelength-dependent absorption and scattering in the water.
Abstract
So far experiments with artificial halos from single transparent crystals have suffered from the lack of apparatus that allows simultaneous rotation around two and three axes. A new setup is presented which overcomes these restrictions by combining electrical as well as pneumatic concepts. This enables reproducible experiments of the most common halos observed in nature and for the first time artificial ring halos from single hexagons rotating around three axes simultaneously. In addition, an old qualitative halo demonstration based on perceived colors of rotating colored areas whose contours represent scattering plots has been reinvestigated and the usually nonsaturated color of artificial parhelia was visualized using a crossed prism method. These new experiments are discussed in the context of all known artificial halo experiments.
© 2014 Optical Society of America
Abstract
Simultaneous visible and long-wave infrared (IR) images of the Moon were used with a simple energy-balance model to study the spatial pattern of lunar surface temperatures. The thermal images were obtained with a radiometrically calibrated, compact, low-cost, commercial IR camera mounted on a small telescope. Differences between the predicted and measured maximum Moon temperatures were used to determine the infrared optical depth (OD), which represents the path-integrated extinction of an elevated layer of wildfire smoke in the atmosphere. The OD values retrieved from the IR Moon images were combined with simultaneous OD measurements from a ground-based, zenith-pointing lidar operating at a wavelength of 532 nm to determine an IR-to-visible OD ratio of 0.50±0.18 for moderately aged wildfire smoke aerosol.
© 2014 Optical Society of America
Visible and invisible mirages: comparing inferior mirages in the visible and thermal infrared
(2015)
Abstract
Visible (VIS)-light and thermal infrared (IR) inferior mirages in the 8–14 μm waveband have been observed simultaneously for the takeoff and landing of various airplanes at distances of several kilometers. Similarities as well as differences between the VIS and IR mirages are discussed.
© 2014 Optical Society of America
abstract
A large part of photonics research and development, as well as commercial applications such as optical data transmission or infrared thermal imaging, occurs in the infrared spectral range between 0.8 μm and 15 μm. However, relatively little material is so far available for experimentally teaching the physics and optics of this spectral range. We report a respective new approach in the near infrared (NIR) range between 0.8 μm and 1.7 μm that allows visualization of a number of fascinating physics phenomena. First, we use the near-infrared sensitivity of silicon-based detectors in rather inexpensive video cameras and digital single-lens reflex cameras by removing the infrared-blocking filter and replacing it with a visible-radiation blocking filter. Second, we utilize modern NIR cameras based on InGaAs detectors. With both camera types we illustrate and explain a number of physics concepts that are especially suitable for curricula in optics and photonics. Examples include the strangely bright appearance of vegetation, contrast enhancement between clouds and sky, the initially surprising differences of optical material properties between the VIS and NIR range, the possibilities of visualizing buried hidden structures and texts, and recent medical applications to locate blood vessels below the skin. © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
abstract
Nature provides many beautiful optical phenomena that can be used to teach optical principles. Here we describe an interdisciplinary education project based on a simple computer model of the colors observed in the famous thermal pools of Yellowstone National Park in the northwestern United States. The primary wavelength-dependent parameters that determine the widely varying pool colors are the reflectance of the rocks or the microbial mats growing on the rocks beneath the water (the microbial mat color depends on water temperature) and optical absorption and scattering in the water. This paper introduces a teaching module based on a one-dimensional computer model that starts with measured reflectance spectra of the microbial mats and modifies the spectra with depth-dependent absorption and scattering in the water. This module is designed to be incorporated into a graduate course on remote sensing systems, in a section covering the propagation of light through air and water, although it could be adapted to a general university optics course. The module presents the basic 1-D radiative transfer equation relevant to this problem, and allows them to build their own simple model. Students can then simulate the colors that would be observed for different variations of the microbial mat reflectance spectrum, skylight spectrum, and water depth. © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Bouncing Poppers
(2015)
Abstract
Toys are known to attract interest in physics and they are therefore often used in physics teaching of various topics. The present paper deals with a simple toy, the so-called “hopper popper,” which, similar to superballs, can be used when teaching mechanics. We suggest some experiments and describe the basic physics of this toy, also providing background information for teachers.
Abstract
A very old and well-known magical trick is the so-called tablecloth pull. A table is covered with a tablecloth, on top of which are certain objects. The task is to remove the tablecloth while the objects—which must not be touched—stay on top of the table. This article describes the physics behind the experiment, and presents examples recorded with high-speed cameras.
Flickering lamps
(2015)
Abstract
Many processes in electromagnetism vary with time. Some of them are well known, in particular those related to line frequencies of 50 Hz or 60 Hz such as fluctuating light output of discharge and incandescent lamps. The flickers of discharge and incandescent lamps have quite different physical principles involved, which are investigated experimentally using high-speed cameras and theoretically using simplified models. The topic is related to other phenomena such as the transient behaviour of phosphor layers covering the screen of oscilloscopes and the time-varying Lorentz force acting on the filament of light bulbs. All studies are well suited for teaching selected aspects of electromagnetism and light at undergraduate level at university.
Light-emitting pickles
(2015)
Abstract
We present experiments giving new insights into the classical light-emitting pickle experiment. In particular, measurements of the spectra and temperatures, as well as high-speed recordings, reveal that light emission is connected to the polarity of the electrodes and the presence of hydrogen.