Attenuation of light by material media: a modeling activity involving three didactic experiments

The attenuation of light by material medium is little discussed in Physics degree courses, especially in experimental classes. In this article, a laboratory activity on this theme is discussed to be conducted with the teaching methodology of Modeling Episodes. The activity comprises three experiments, explored from the perspective of Didactic-Scientific Modeling in order to make sense specially of the concept of expanding a theoretical model. The problematization of the investigations is derived from discussions about how water pollution influences the photic zone of the oceans. In order to answer the questions raised, it is necessary to expand the model of propagation of light in a vacuum, including processes of absorption and scattering of light by matter, which was done through Beer-Lambert’s law. With the proposed practices, it is sought to highlight how a theoretical model of Physics can be improved through the inclusion of previously disregarded effects, thus expanding its domain of validity and predictive capacity, enabling the conduct of discussions on the nature of the scientific modeling process in experimental classes. In order to empirically contrast the expanded theoretical model, three experiments were done, each one of them in defferent media (glass, solution of water + table salt and smoke), using lasers and lamps as light emitting source. Particular emphasis was given to the role of the idealizations assumed in the process of construction, use and validation of the constructed model, seeking to evaluate, in each situation, the consequences of these simplifications of reality. From the results of the experiments, it is concluded that: i. the expanded theoretical model describes the attenuation of light intensity in material media with good precision, and ii. the pollution of the oceans can contribute to increase the extinction of sunlight, thus decreasing its penetration in the photic zone, reducing the photosynthesis rate of the algae. This article can contribute to the field of Physics teaching, not only bringing an innovative laboratory teaching activity, but also serving as a reference for those teachers interested in the practical application of modeling activities, in particular those focused on the expansion of a theoretical model.

Keywords
Beer-Lambert Law; Attenuation of light; Scientific model; Modeling Episodes