November 2025
Volume 93, Issue No. 11
Color and thickness of soap films
The subject of soap films and their color is revisited. Following an educational path through wave optics, an account is provided for the color pattern of the films as related to the film thickness, spectral bandwidth of the light source, and sensitivity of the detector. A simple experimental setup is described, where a soap film is observed under illumination from an extended source and an image of the color pattern is recorded with a digital camera. The image data are analyzed to determine the thickness profile of the film, where the film thickness is obtained to within an accuracy of 3%. The data processing method is described, and examples of the determined film profiles are given. The presentation is suitable for students in their first years of college, but, if properly adapted, can also be appropriate for secondary school classes.
EDITORIAL
In this issue: November 2025 by John Essick; Jesse Kinder; Raina Olsen; Beth Parks; B. Cameron Reed; Donald Salisbury. DOI: 10.1119/5.0304025
AWARDS
2025 Paul W. Zitzewitz excellence in K-12 teaching award: Getting by with a little help from my friends by Bree Barnett Dreyfuss. DOI: 10.1119/5.0305307
PAPERS
Bending time: The tubular water clock by Keith Zengel; Hong-An Luu; Samuel Stollman. DOI: 10.1119/5.0279843
Editor's Note: Water clocks go back to antiquity, using a container shaped so that its water level rises or falls at a constant rate to mark the passage of time. This paper describes an experiment where a flexible clear plastic tube is bent at an angle to the vertical; with the correct bending, a steady flow rate can be achieved. The paper presents the mathematical analysis of the flow and experimental results obtained with smartphone videos. The experiment can be conducted by students in a single laboratory session, during which they can test different tube shapes. Appropriate for both introductory and advanced students.
Matter, interaction, and change: A framework for an integrated approach to teaching introductory physics by Eugene Hecht. DOI: 10.1119/5.0173764
Editor's Note: Our understanding of matter and mass has fundamentally changed several times over the last few centuries. The concept of atoms was introduced by ancient Indian and Greek philosophers, developed more formally by Boltzmann in the late 19th century, and was only considered proven by Perrin in 1908. Thomson showed that electrons were particles in 1897, while Rutherford showed the same for nuclei in 1911. This prompted Bohr's “solar-system” atomic model, which lasted only until de Broglie proposed the concept of “matter waves” and Schrodinger's 1925 wave equation, which supplanted Bohr's heuristic. So matter is particles, but particles are actually waves. Meanwhile, at the other end of the length scale, relativity was being invented, with Einstein showing in 1905 that mass was actually energy. And yet most introductory textbooks give centuries-old classical definitions such as “mass is the quantity of matter,” while thermodynamics, quantum, and relativity textbooks often develop them in very different ways. How should these foundational concepts be properly taught in early physics courses and across fields? This paper discusses the philosophical issues and proposes a consistent scheme.
Energy needed to propel a tiny spacecraft to Proxima Centauri by C. J. Umrigar; Tyler A. Anderson. DOI: 10.1119/5.0263298
Editor's Note: Could we send a spacecraft to another star? Sending a person to the nearest star is the stuff of science fiction. But it turns out that sending a 2 g spacecraft, while not possible with current technology, is perhaps within reach. To do this, we could shine a powerful laser on the spacecraft's sail, accelerating it to perhaps 0.2c. How much light energy would be required? This paper shares the details that will allow students in an introductory course on special relativity to calculate the answer.
A Lorentz force law with velocity differences by Rinke J. Wijngaarden. DOI: 10.1119/5.0261338
Editor's Note: Many students struggle to understand the electric fields associated with moving magnets, even though the Lorentz force law applies in any reference frame. This article shows how common errors arise and presents a simple, intuitive reformulation of the Lorentz force law that helps avoid such mistakes. The examples and “paradoxes” are well-suited for introductory and intermediate electrodynamics courses, offering opportunities for classroom discussion and fresh insight into the fields of moving magnets.
A very simple derivation of the periastron advance to all post-Newtonian orders of perturbation in Schwarzschild geometry by Steven A. Balbus. DOI: 10.1119/5.0273160
Editor's Note: Explaining the advance of orbital perihelion was one of the major early successes of the theory of general relativity, and it remains a fixture of GR courses at the advanced undergraduate and graduate level. However, this explanation has not been something we could share in earlier physics courses. This paper presents a simple mathematical technique that will allow instructors in lower level undergraduate courses to share an interactive analysis of an orbital perihelion advance that can be calculated with arbitrary accuracy. One immediate noteworthy application is the analysis of this advance for stars orbiting the black hole at our galactic center.
Frequency dependence of the displacement current density by Toshio Hyodo. DOI: 10.1119/5.0237905
Editor's Note: The standard teaching sequence for electromagnetism is elegant, but it can easily lead to confusion. Students usually learn, by considering a charging capacitor, that a changing electric field induces a magnetic field. Then they learn that a changing magnetic field induces an electric field, and that these induced fields result in electromagnetic waves. But they usually don not re-consider the charging capacitor in light of the induced electric field. This paper provides a clear explanation of displacement currents in a capacitor in an AC circuit, examining the relative sizes of the Coulomb field and the induced field in creating displacement currents. It will make a nice addition to upper-level electromagnetism courses.
ADVANCED TOPICS
Quantum description of the asymmetric top using average and relative coordinates in an operator-based formalism by E. Toprak; Dominika Zgid; J. K. Freericks. DOI: 10.1119/5.0258534
Editor's Note: This paper shares an operator-based approach to calculating the rotational spectra of asymmetric tops, clarifying the relationship between the lab frame and the body-fixed frame. This approach could be used in a graduate-level course on quantum mechanics or molecular spectroscopy, and suggested student activities are included in the paper.
NOTES AND DISCUSSIONS
What is the maximum radius of cold planets? by David Garfinkle; Alberto G. Rojo. DOI: 10.1119/5.0226963
Editor's Note: Planets with no internal or external sources of energy have maximum radii on the order of that of Jupiter. What physics dictates this? This paper develops a variational model of a pure-hydrogen planet very similar to that used to estimate the Bohr radius for the hydrogen atom to determine how this maximum depends on electrostatic and gravitational constants; results are in good agreement with more sophisticated models. Appropriate for students familiar with modern physics and astrophysics.
INSTRUCTIONAL LABORATORIES AND DEMONSTRATIONS
A Raspberry Pi-based compact, portable, inexpensive IR viewer for the advanced optics undergraduate laboratory by Chanakya Pandya; Yuki Brown; Caden McCollum; Reese Tyra; Nikolus Miller; Muhammed Khan; Catherine Kasicki-Rodriguez; Sara McGinnis; Samir Bali. DOI: 10.1119/5.0251130
Editor's Note: In this paper, the authors present a novel way to construct an infrared viewer using inexpensive components and give step-by-step instructions for assembly and operation. Comparison of this homebuilt viewer's performance to a commercial IR viewer and night vision goggles is given, along with laboratory applications in which the IR viewer can be used, including acquiring photos of alkali-atom fluorescence and imaging an infrared laser beams passage through tabletop optics. Students can construct this IR viewer as an educational project and the assembled device is well-suited for use in advanced instructional and research laboratories.
Color and thickness of soap films by Marcello Carlà; Giuseppe Molesini; Samuele Straulino DOI: 10.1119/5.0275602
Editor's Note: This paper describes a thin-film interference experiment on soap films under both monochromatic and polychromatic light illumination. Using a simple experimental setup, data are acquired by a color digital camera, yielding red-, green-, and blue-filtered images of interference fringes. A method based on elementary optics concepts is presented and used to analyze the acquired data, with the primary purpose of accurately determining the thickness profile of a vertically oriented soap film. A more in-depth analysis method is also given in the Appendix. The authors have provided a resource that can be adapted to a variety of levels in physics education, from secondary school and introductory college classrooms to the advanced instructional laboratory.
Additional Resources
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