Presentations

Active Learning

Phone Cord Lab. Deborah Lojkutz and Ann Brandon, Joliet West High School, Joliet, IL 60453. We will present a lab activity that uses a simple phone cord to allow students to investigate the nature of waves and discover for themselves the relationships that exist between a wave's wavelength, frequency, period and speed.

Introductory Lab on the Structure of DNA. Steven Daniels and Cherie Bibo Lehman, Eastern Illinois University, Charleston, IL 61920. An introductory diffraction lab will be presented. This lab delves into the search for an understanding of the structure of DNA. The history surrounding the discovery of the double helix structure will be presented. A model representing the helical structure in a diffraction experiment will be demonstrated. This introductory lab is appropriate for life science students to experiment with practical applications of optical diffraction showing x-ray crystal diffraction. It will engage the student with the important result that it explains the discovery and measurement of the structure of DNA.

Teaching Methods

Assessment Guiding Instruction in Physics. Robert E. Lang, Glenbard South High School, Glen Ellyn, IL 60137. The presenter will share methods he uses in his physics classes to assess student understanding and guide his instruction. Topics include pre/post-assessments, grouping of students, formative assessments, lab assessments and summative assessments. The presenter will begin with explaining the framework of correcting student misconceptions in physics. He will then talk about how he uses pre-assessment data to group students according to their needs. Then he will discuss how he uses multiple formative assessments to improve student understandings. The presentation will conclude with a discussion of how summative assessments (unit exams) are used to guide instruction after the unit is over. Questions, comments, suggestions are welcome.

Discussion Topics in an online Introductory Astronomy Course. Noella D'Cruz, Joliet Junior College, Joliet, IL 60431. Joliet Junior College offers Introductory Astronomy (ASTR 101) for non-science majors in the online format in addition to the face-to-face-format. At the Fall 2010 ISAAPT meeting, I talked about how I developed the online version of this course. The online setting offers different opportunities for student interaction compared to the face-to-face format. Usually students interact with each other using discussion boards. They are encouraged to include links to websites and attachments in their discussion board posts. At this meeting, I would like to present discussion board topics that I have used in my online offerings of ASTR 101, and how students have responded to them.

Seeing the Invisible. James Rabchuk, Western Illinois University, Macomb, IL 61455. This past year I taught a 1 credit hour seminar course for Honors students at WIU, which I called "Seeing the Invisible." This course grew out of my experiences in helping teach our capstone science education course. I structured the course around a simple question: How can we obtain reliable knowledge about things we can't see? The corollary question is: Why do we need to worry about invisible things? I drew on literary sources, as well as science and popular literature, and built each class around demonstrations or experiments that explored "invisible" phenomena such as Brownian motion, high speed motion, sound, gravitation, cosmic radiation, infrared radiation, etc. I'll present a few of lecture topics, some of the readings and some of the student responses to the class. I'll also try to draw some conclusions about the need for such a course, not just for honors students or teacher ed students, but also for physics majors.

Using Angry Birds to teach Computational Thinking. Daniel DuBrow, Evanston Township High School, Evanston, IL 60204. Working with Jason Hwang, a graduate student at Northwestern University, we built upon others' lessons with Angry Birds. In this lesson, we challenge students to calculate the acceleration due to gravity, "g" in Angry Birds world. We use this as an intermediate lesson on the way to computational thinking exercises such as programming physics simulations.

Renewal of the Advanced Physics Laboratory Course. Brian Davies, Western Illinois University, Macomb, IL 61455. Many physics majors take an advanced laboratory course intended as an introduction to modern experimental physics. Few realize how problematic this course has become in the current curriculum. To address this issue, an organization of lab instructors has been formed to address the basic rationale and role of the course, serve as a mutual-aid society, and promote the importance of this course in the undergraduate curriculum. The Advanced Laboratory Physics Association (ALPhA) will sponsor a summer meeting to address a range of issues (jointly with APS, AAPT, and others). Audience members will be asked about the value of their own advanced lab experience.

The Physics of Theatre: A Scenario-Based Interdisciplinary Course. Eric C. Martell, Verda Beth Martell, Millikin University, University of Illinois Urbana-Champaign, Decatur, IL 62522. The Physics of Theatre Project was started in 2003 with the primary goal of helping theatre technicians utilize basic physics concepts in scenery design and construction. One of the outcomes of the project has been a textbook and accompanying course, taught for the first time in complete form this year at Millikin University. The course was taught in a truly interdisciplinary way, using faculty, equipment, and facilities from both Physics and Theatre departments, and focused on the direct application of physics concepts to theatrical scenarios. The final "exam" for the course required the students to design a series of scenarios based on realistic theatrical situations, such as motor-driven systems, rigging, and turntables, and then perform the necessary calculations to analyze the scenarios for practicality and equipment needs. Outcomes of the course as well as impacts on future coursework will be discussed.

Effect of Introductory Physics on Statics Concept Inventory (SCI) Scores. Tom Carter, College of DuPage, Glen Ellyn, IL 60137. The engineering community has its own organization of educators, the ASEE, and its own set of conceptual inventories, including the Statics Concept Inventory (SCI). I will provide data on the effect of taking an introductory physics class on students' SCI score. I will look at their scores both at the end of the physics class and the end of the engineering statics class. This may assist in answering the question: "Should physics be a pre-req for the engineering statics class?"

Physics and the K-12 Frameworks. Tom Foster, Southern Illinois University Edwardsville, Edwardsville, IL 62026-1654. In March, 2012, the National Research Council released "A Framework for K-12 Science Education." Intended to replace the existing National Science Education Standards and the AAAS Benchmarks for Scientific Literacy, the Frameworks make sweeping changes to how physics is conceptualized. The Frameworks will be used by Achieve Inc. to create standards and assessments intended for use in over 40 states. Given the scope and significance, every physics and chemistry teacher should be aware of how K-12 education will be changing.

Using Student-generated Screencasts for Assessment. Andrew Morrison, Joliet Junior College, Joliet, IL 60431. Many physics faculty are starting to use screencasts to assess homework and lab reports from their students. The screencasts are recorded by students who are required to explain each step of the assigned work. The mechanisms for assigning screencasts, recording them and using them as a tool for assessment will be discussed in this presentation. I will also discuss the successes and challenges I've faced with implementing screencasting into my introductory physics class this semester.

Demonstrations

Research

Negative Refractive Index in Atomic Media. Amitabh Joshi, Eastern Illinois University, Charleston, IL 61920. We discuss the possibility of generating negative index of refraction in an atomic medium. The medium is consisting of five energy levels in K-type configuration and interacting with four electromagnetic fields. Both electric and magnetic dipole transitions are considered. The calculation of refractive index is provided by solving density matrix of atom-field system at finite temperature and obtaining coherence parameter. The interference arising due to the nearby decaying levels is also included in the model which is found to be a very important mechanism in producing negative refractive index in the atomic medium. The atomic system exhibits negative electrical permittivity as well as negative magnetic permeability simultaneously, and consequently shows the negative refractive index for certain range of physical parameters characterizing the system.

Pulsing and Focusing an Electron Beam. Pengqian Wang, Western Illinois University, Macomb, IL 61455. Electron impact dissociative ionization of molecules is a useful method in exploring the structure of molecules and their interaction with electrons. An economic electron gun is constructed in our lab to provide an electron beam source needed in the experiment. The electrons are generated from a heated tungsten filament in a high vacuum. They are accelerated to a few hundreds of electron volts. The electron beam is pulsed with a duration of about 100 ns. It is focused to the molecular target by an ion optical system. The trajectory of the electrons is simulated by a commercial software. The electron gun has been used in our lab to impact and ionize atoms and molecules, and the resulted mass spectra have been measured.

Student Research Symposium

Testing the Effects of a Rotating Magnetic Field. Brandon Emerson, Dustin MacDermott, and James Rabchuk, Western Illinois University, Macomb, IL 61455. The objective is to test competing models of how electromagnetism works in a rotating coordinate system. The basic question: whether or not a rotating charged solenoid will produce a potential difference on concentric capacitors located inside or outside the solenoid. Using computer simulation software we will calculate the expected induced radial electric field predicted by the competing models. We intend to detect these fields by looking for a potential difference across the capacitors. They will be connected with wire while the solenoid is rotating at a set angular velocity. The connection is broken before bringing the solenoid to rest, and a high impedance voltmeter is used to measure any potential difference. Our hope is to distinguish between the competing theories and determine which best explains the observed results. In this talk I will present the results of our simulations.

The Theoretical Mysteries of Axially Rotating Solenoids. Dustin MacDermott, Brandon Emerson, and James Rabchuk, Western Illinois University, Macomb, IL 61455. We seek to design and carry out an experiment that will distinguish between three competing theoretical models of how electromagnetism behaves in a rotating coordinate system. In particular, we look for the possibility that a powered, rotating solenoid will induce a radial electric field in the lab frame. The standard model prediction is there will be no radial electric field when rotating a solenoid. The rotating flux model predicts the maximum radial electric field to be induced inside the solenoid. The third model invokes relativistic effects from differences in velocities of the electrons and ions in the solenoid. This model predicts charge build up on the wire of a rotating solenoid producing a radial electric field of greatest magnitude outside the solenoid. In this talk I will discuss the quantitative predictions from these models, as well as the proposed experimental design.

Detecting Analytes using Nanocantilever-based Biosensors. Patrick Snyder and Amitabh Joshi, Eastern Illinois University, Charleston, IL 61920. Trends in healthcare research have recently shifted focus from simply treating illness to prevention and early detection. Biosensors are becoming increasingly important for detecting illnesses early. These homeostatic imbalances that are telltale signs of illness, can only be measured on the nanoscale. A nanoscale biosensor most suitable for the monitoring of such imbalances is based on the use of nanocantilevers. These microscopic 'diving boards' are covered with probes that serve as a type of biological magnet that attracts a specific amino acid, enzyme, or protein in the body. These binding probes on the silicon cantilever attract target particles, and as the analyte collides and binds with the cantilever, it causes these boards to oscillate with a certain frequency and amplitude. In this research work we have interpreted the amplitude and frequency of oscillations exhibited by the cantilever. This was done to obtain a better quantitative understanding of nanocantilever biosensors.

Super-Resolution at the Nanometer Scale: Using Simulated Emission Depletion Microscopy to Break the Diffraction Limit, Oluwatobi Olorunsola and Kishor Kapale, Western Illinois University, Macomb, IL 61455. For many years, applying microscopy with focused light meant that details smaller than half the wavelength of light (200 nm) could not be resolved. Today, it is known that using conventional optics it is possible to image at least fluorescent samples with a level of detail far below the diffraction limit. Stimulated Emission Depletion (STED) microscopy and newer far-field optical approaches can provide resolutions better than 20 nm, and in principle are able to resolve molecular detail. While the diffraction barrier has motivated the invention of electron, scanning probe, and x-ray microscopy, in the life sciences 80% of all microscopy studies are still performed with lens-based (fluorescence) microscopy. In this presentation, I will discuss novel physical concept of STED, which radically breaks the diffraction barrier in focused fluorescence microscopy. The strategy in this concept exploits selected molecular transitions of the fluorescent marker to neutralize the limiting role of diffraction.

Charged Particles in a Neutral Line Magnetic Field: Final State Sensitivity.  Jamie Svetich and Richard Martin, Illinois State University, Normal, IL 61790-4560. Interactions between plasma particles and Earth's magnetic field are relevant to magnetic storms, which affect the aurora, radio communications, and the power grid. Observations indicate a region in the Earth's magnetotail is important in the dynamical processes involved. In this talk we will concentrate on particle dynamics in a magnetic neutral line field, where the magnetic field goes to zero along a line across the magnetotail. Previous work has shown scattering in a current sheet field has fractal behavior and we would like to see if the neutral line exhibits the same properties. The motion is known to be chaotic for some parameters and I will investigate final state sensitivity after the particles scatter off the neutral line region.

Charged Particle Dynamics in a Neutral Line Magnetic Field: Energy Resonance Behavior. Connor Brennan, Illinois State University, Normal, IL 61761. Connor Brennan, Dr. Richard Martin. The magnetic neutral line field in the tail region of the earth's magnetosphere causes particles to behave in a very interesting manner. The magnetosphere is the magnetic cavity carved out of the solar wind by the earth's internal magnetic field. The magnetic field has a long tail extending from the side of the earth opposite the sun. In this tail, plasma instabilities, brought on by solar winds "hitting" the magnetosphere, are thought to cause magnetic storms and the Aurora Borealis. We research the basic plasma physics behind these instabilities using computer simulations of charged particles, looking for signatures that could potentially be observed on spacecraft. We use a program to calculate the trapping time of particles in the neutral line field, as a function of energy. We examine this data for resonances.

Concentration-dependent Nonlinear Optical Response of Silver Nanoparticles in Castor Oil. Trevor Smith and Abdullatif Hamad, Southern Illinois University Edwardsville, Edwardsville, IL 62026. In recent years, a large amount of research has been devoted to understanding the optical characteristics of composite systems consisting of a host material and nanoparticles of a different material. In particular, nonlinear optical processes, which become manifest in the presence of intense laser irradiation, often produce startling effects that are enhanced by the addition of metallic nanoparticles. Here, we present measurements of the thermally-induced change in the refractive index of a silver nanoparticle/castor oil system at various particle concentrations. The samples were prepared via laser ablation, and the refractive index change was measured using a closed-aperture, pump/probe scanning technique known as "x-scan". The results of this study and others like it may have important applications in the development of photonics devices.

Initial Preparations of a Large-scale Astronomical Observatory.. Cody Dirks, Southern Illinois University - Edwardsville, Edwardsville, IL 62025. Well-executed up-front design is extremely important for the efficient operation of any observatory. Preparing an observatory must take into account the local environment and atmospheric conditions, which may have a considerable effect on the necessary design and operation of the telescope. Without taking these conditions into consideration and making sure they are properly negated, images are often too dim or out of focus to use, and spectroscopic data may appear shifted and/or filtered. We describe the design and components that have been chosen for Southern Illinois University - Edwardsville's new observatory and the reason for selecting them. In addition, we explain the timeline of construction, with a focus on the integration of systems necessary to make the observatory operate smoothly and allow for much more efficient data acquisition. Finally, we investigate future plans to automate data collection systems for more accurate data runs.

Computer Simulations of Wetting for Materials of Varying Wettabilities. Hannah Tanquary and Jie Zou, Eastern Illinois University, Charleston, IL 61920. Wetting is an important topic in science and has many applications in engineering. Wetting refers to a phenomenon where a liquid (such as water) keeps contact with the surface of a solid (such as glass). Different solids have different wettabilities. In terms of physics and chemistry, wetting is the result of intermolecular interactions (adhesive forces) between a liquid and solid and those (cohesive forces) within the liquid. Theory has suggested that wettability depend on the relative strength between the adhesive and cohesive forces. The purpose of this project is to test the above theory by performing detailed computer simulations. Our simulations will be based on a computational method known as the Monte Carlo simulation. Specifically, in our simulations, we will investigate how the contact angle (a measure of wettability) depends on the relative strength between the adhesive and cohesive forces.

Computational Near Earth Asteroid Search. Tyler Linder and Jie Zou, Eastern Illinois University, Charleston, IL 61920. NASA has a Congressional Mandate to find and track all Near Earth Objects (NEOs) greater than 1 km in size. A NEO is any object that comes within 1.3 AU (1.9 x 10¹¹ m) of Earth. Near Earth Asteroids (NEAs) are a large portion of the NEO population. Asteroids 1 km in size and larger have the potential to cause severe damage to Earth if an impact ever occurs. The Astronomical Research Institute (ARI) of Westfield, Illinois has been the world's leading follow-up (tracking) observatory of NEAs for the past three years. Oftentimes unknown asteroids are discovered while conducting these follow up observations. However, the techniques ARI uses in the follow-up process reduce the likelihood of discovering unknown NEAs. The proposed research project is to develop a computer algorithm that will search ARIs images for unknown asteroids. This computer algorithm will increase the chance of discovering unknown asteroids by a factor of 100.

Application of Atomic Coherence Effects to Super-Resolution. Ademola Jinadu and Kishor Kapale, Western Illinois University, Macomb, IL 61455. Interaction of light with two-level atoms is well understood through the phenomena of absorption, spontaneous emission, and stimulated emission of light. However, real atoms always have more than two levels and this multi-level structure can be exploited for variety of interesting applications. In general, interaction of light with multi-level atoms generates interference effects that are commonly termed as atomic coherent effects, for example coherent population trapping and electromagnetically induced transparency. Through these atomic coherent effects one has a controllable handle on the response of an atomic medium to the light fields that are incident on it. In this presentation I will present the interaction of three level atoms with two light fields and how it can cause emergence of structures smaller than the wavelength of both the light fields that the atom is interacting with. These ideas are applicable to atom localization, nano-lithography, and microscopy beyond the Rayleigh limit.

Fluorescence in Silver-doped Lead Borate Glass. Akinloluwa Olomoroti, Saisudha B. Mallur and P.K. Babu, Western Illinois University, Macomb, IL 61455. We carried out Pb²⁺ fluorescence measurements in lead borate glasses and studied the effect of adding Ag into the base glass. Lead borate glasses containing Ag (0, 1, 2 and 3 mol%) were prepared by the usual melt quench method. The prepared glasses were then annealed near the glass transition temperature (400^oC) at 5, 10, and 20 h. Fluorescence spectra of all these samples were obtained using different excitation wavelengths. In general, Pb²⁺ monomers are expected to have emission at wavelength less than 400 nm. However, no emission in this region was observed due to the base glass absorption. The emission observed at 450 nm is attributed to ³P₁ → ¹S₀ transition of Pb²⁺ ions in dimer centers. Addition of Ag enhances the Pb²⁺ luminescence intensity at 450 nm which also shows an increase with the annealing time. The possible mechanisms for the fluorescence enhancement in the present glass could be the energy transfer from isolated Ag particles and local field effects due to the difference between the dielectric functions of the glass matrix and the silver particles.

Influence of Silver and ZnSe Nanoparticles on Electric-dipole and Magnetic-dipole Transitions of Eu³⁺ doped Lead Borate glass. Stewart Ferrell, Mark S. Boley, P.K. Babu and Saisudha B. Mallur, Western Illinois University, Macomb, IL 61455. Fluorescence properties of Eu³⁺ doped lead borate glasses containing either silver or zinc selenide nanoparticles (NPs) were investigated. Lead borate glasses containing Ag (0 and 3 mol%) and ZnSe (0 and 3 mol%) were prepared by the usual melt quench method. The prepared glasses were then annealed near the glass transition temperature (400^oC) at 5, 10, and 20 h. The effect of nanoparticles can be clearly seen on the Eu³⁺ fluorescence transitions in the range from 570 to 720 nm. Electric-dipole and magnetic-dipole transitions that originate from the Eu³⁺ level ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ respectively, exhibit changes in fluorescence intensity due to the presence of NPs. The possible mechanisms for the fluorescence enhancement in the present glass could be the energy transfer from isolated NPs and due to the changes in the structural environment of the Eu³⁺ ion induced by the presence of the NPs.

ESPI in Two Dimensions. Josiah D. Kunz, and J. Scott Steckenrider, Illinois College, Jacksonville, IL 62650. Electronic speckle pattern interferometry, or ESPI, is a method for rapid quantification of surface motion with sub-micron resolution which is applicable to any opaque rough surface. This type of testing has proven beneficial in many industries as a quality assurance test, particularly in the fields of aerospace and automotive engineering. In the current work involving thermal expansion of a surface, the surface motion was primarily in-plane, but occurred in two dimensions. Consequently, ESPI in two dimensions was developed to evaluate the sample. To analyze this thermal expansion, a pre-existing one dimensional ESPI system was modified to simultaneously evaluate motion in both vertical and horizontal directions through polarization multiplexing. The result was an adaptable interface that allowed the user to quantify two-dimensional in-plane deformation with ten-nanometer scale resolution. These results from a particular controlled sample will be presented.

Using Lock-in Detection to Measure Faraday Rotation of Nanoparticle Composites. Christopher Carr, Southern Illinois University Edwardsville, Edwardsville, IL 62025. Faraday rotation describes the rotation of the plane of polarization of light traversing a medium immersed in an external magnetic field. These rotations are very small for most materials, and accurate measurements of these rotations for materials a few centimeters in length can be quite difficult. Through the use of a lock-in amplifier these values can be measured quite accurately, even in the presence of large amounts of noise. Using an experimental apparatus employing a lock-in amplifier we took measurements for materials with well known Verdet constants to test the accuracy of our experimental technique. These measurements proved to be quite accurate when compared with published data. Currently we are using this apparatus to investigate Faraday rotation in nanoparticle composite samples, specifically CoAg nanoparticles and Ag nanoparticles in castor oil. The effective length of these samples is on the order of 100 nm making Faraday rotation much harder to detect.

Calculation of Perturbed Excited States of Argon. Jeffrey White, Southern Illinois University Edwardsville, Edwardsville , IL 62026. Recent work suggests that collisional ionization in rare gas atoms is primarily a two-step process in which the outer electron is first promoted to an excited state and then subsequently ionized. This presentation describes theoretical work currently being done to examine excited states in argon atoms. Of particular interest is the effect of nearby ions on excited state energies. A numerical Schroedinger equation solver is used to calculate these energies. This program uses the finite-difference time-domain method to approximate Schrodinger equation solutions for an arbitrary potential shape. In this case, a psuedopotential is used to simulate the lower 17 electrons in an argon atom while a nearby ion is simulated by the Coulomb potential from a point charge. This work aims to determine the validity of the gaseous atomic excited states in a cluster environment and is a first step in understanding important quantum mechanical effects in laser-cluster interactions.

Fermion and Boson Pair Creations. Matthew Ware, ILP Theory Unit and Department of Physics, Illinois State University, Normal, IL 61790-4560. Using numerical solutions to quantum field theory, the creation of particle-antiparticle pairs from the vacuum due to a strong external localized electric field is explored. It is shown that the presence of an incoming fermion can suppress the creation of fermion-antifermion pairs by the external field, and the consequences of this fact for the so-called Klein paradox are discussed. In contrast to the fermionic system, an incoming boson enhances boson-antiboson creation rates through the process of stimulated emission. It is shown that this leads to an exponentional self-amplification of boson pairs in a supercritical potential well.

Causality in Quantum Mechanics and Quantum Field Theory. Benjamin Shields, ILP Theory Unit and Department of Physics, Illinois State University, Normal, IL 61790-4560. Superluminal motion and its connection to causality is explored in the context of quantum mechanical systems. It is shown that, while the mean velocity of a quantum wave packet may be less than c, certain portions of the wavepacket may still spread superluminally. Criteria are developed that can be used to determine if a wavepacket is spreading superluminally, and these criteria are used to show that the relativistic Schrodinger equation is non-causal and that up to 8% of the wavepacket may violate causality. It is also verified that the Dirac and Klein-Gordon equations are causal, and extensions of this work to quantum field theories are briefly discussed.

Space and Time Correlation Functions for Dressed Bosonic Vacuum States. Andrew Vikartofsky, ILP Theory Unit and Department of Physics, Illinois State University, Normal, IL 61790-4560. We analyze the spatial and temporal dynamics of virtual particles in the vacuum states of one-dimensional φ2- and φ4-model systems. The properties of the vacuum state for the φ2-system can be found analytically, which allows us to compute all spatial and temporal correlations exactly. The momentum distribution of the vacuum virtual pairs is examined as well as the spatial and temporal correlations between virtual particles for both systems. We argue that almost all of the vacuum's properties can be explained in the usual particles terms.

Reconstruction of Objects in Turbid Media. Ben Rogers, ILP Theory Unit and Department of Physics, Illinois State University, Normal, IL 61790-4560. Methods for using laser light to detect multiple unknown objects inside of random media are discussed. The measured shadow pattern can be decomposed into its component "eigenshadows" by diagonalizing the covariance matrix. It is shown that the resolution of objects that are upstream and closer to the incoming laser light is lower than the resolution of objects further downstream, and techniques to improve the resolution of objects are also discussed. Finally, a method of detecting objects by minimizing χ² while scanning through various positions for the object is introduced.

Test of a Minimization Scheme for Imaging with the Presence of Noise. Brandon Graybeal, ILP Theory Unit and Department of Physics, Illinois State University, Normal, IL 61790-4560. Simulations for detecting objects inside of biological materials by using laser light are presented. Our method uses the weighted shadow patterns of objects at fixed locations to "mask" one section of the detection region, so that objects in the unmasked region can be located by scanning through various locations and minimizing χ². It is shown that when random noise is introduced, the method is very robust when only a single object is present in the medium but that detection of multiple objects can be very sensitive to noise.

Multi-runner Algorithm for Image Inversion. Alexander Su, ILP Theory Unit and Department of Physics, Illinois State University, Normal, IL 61790-4560. A technique to locate rods inside of a highly scattering medium is presented. The detected shadow pattern which is cast by an incoming laser beam is used to reconstruct the location of the rods. The method is based on a scanning technique where a weighted, double-rod configuration scans through the medium and the weights of the rods' shadows are fitted to the observed shadow pattern. It is shown that one of the fitting weights vanishes when the location of the desired rod is found. Finally, it is demonstrated that the method is less sensitive to noise than a prior method based on minimizing χ².

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