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We explore alternative ways to assign a measure to dynamic speckle patterns produced by different phenomena. Both the power spectrum and the autocorrelation of the intensity are considered. First, we describe an experimental study of the statistics of the Fresnel speckle patterns produced by illuminating a diffuser with laser light as a phenomenon where some control can be maintained on the dynamic variables. The diffuser rotates in a plane with constant angular speed. A correlation is established between some parameters of the autocorrelation or the power spectrum of the intensity fluctuations of the speckle patterns and the tangential speed corresponding to different distances from the rotation axis. A scale correction is suggested to estimate the number of points of the autocorrelation function that should be included in the calculations before speckle crosstalk starts. The defined parameters are then applied to characterize the activity of the dynamical speckle produced in the drying of paint. This is considered as a starting point for the understanding and assessment of more complex situations including biological activity. © 2000 Society of Photo-Optical Instrumentation Engineers. [S0091-3286(00)01006-0] Subject terms: dynamic speckle; autocorrelation function; drying of paint.Paper 990004

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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.