An Inexpensive Interferometric Setup for Measuring Microscopic Displacements

Abstract

In an interesting article published in an issue of The Physics Teacher, Reichert1 gives some didactic examples about static friction force between a plastic block and a wooden plane on which it rests. To explain the experiments, he uses a simple model based on a microscopic “elastic band” that connects the atoms of both surfaces. Reichert remarks that “the block moves, albeit a microscopic distance,” and that it would be helpful if the student could see these displacements. In another paragraph he states that “measuring it (displacement) requires delicate and expensive optical instruments.” Effectively, a measurement of such small displacements generally requires interferometric devices. At our university, we teach basic physics and we are aware of the difficulties that beginners have grasping the concepts involved in static friction force. At the same time, as our research field is related to optics metrology, we could not ignore Reichert's statement. Could we design an experimental device to measure the microscopic displacement referred to by Reichert, keeping it inexpensive and easy to implement? Incidentally, in the same issue of The Physics Teacher, Sawicki2 gives an excellent example of how, with a few common elements, a simple experiment of interferometric measurement can be put within students' reach. In this paper, we suggest the use of a simple interferometric device, built with very common and inexpensive elements, and describe an experiment on static friction force in which the instrument is applied to measure microscopic displacements.