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Smartphones are widely available and used extensively by students worldwide. These phones often come equipped with high-quality cameras that can be combined with basic optical elements to build a cost-effective DIY spectrometer. Here, we discuss a series of demonstrations and pedagogical exercises, accompanied by our DIY diffractive spectrometer that uses a free web platform for instant spectral analysis. Specifically, these demonstrations can be used to encourage hands-on and inquiry-based learning of wave optics, broadband versus discrete light emission, quantization, Heisenberg’s energy-time uncertainty relation, and the use of spectroscopy in day-to-day life. Hence, these simple tools can be readily deployed in high school classrooms to communicate the practices of science.

This paper is the second in a series of published solutions¹ discussing problems of the Ortvay Rudolf international competition.

The problem treated below is a simple exercise about grasping the fundamental aspects of a given phenomenon described within a qualitative “verbal” report and applying the principles learned in classical mechanics and geometric optics in order to explain its mechanism. The most important part of such problems is the interpretation of the phenomenon at hand, which naturally has a certain degree of vagueness, often allowing multiple scenarios. Similar tasks are often encountered, i.e. at the conceptual level of engineering and design.

In this case, the problem can be interpreted most straightforwardly as the description of an optical phenomenon in which a free-falling observer sees (presumably) its own delayed optical image. Here, we will focus on one concrete solution, the calculation of which does not use mathematical techniques beyond those expected of first-year university students.

We describe a simple experimental apparatus which allows the quantitative study of the classical analog of a quantum eraser. The apparatus consists of a laser diode, a thin wire, two polarizers providing the which-way information, and a third polarizer which erases that information. The experimental results are analyzed with the aid of a digital camera and the Tracker application, and are compared with a novel classical computation of the interference patterns, which is presented as well.