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The goal of the laboratory exercises is to give students a physical insight into the phenomena under study and to introduce them into the actual use of laboratory equipment. With this aim, each experiment is envisaged and designed by the professor to reinforce an important physic concept. In addition, students are provided in advance with a Lab Procedure containing a theoretical introduction and detailed explanations of measurements and procedures to carry out. Hence, students are expected to be acquainted with the basic knowledge of an experiment before attending the laboratory. However, teaching experience in lab shows that students generally do not come to the lab prepared, they have not previously read the lab procedure and do not have knowledge of the main ideas of an experiment before coming to the laboratory. They assume a passive role and just follow the steps and indications given in the lab procedure.

These reasons have motivated us to consider a different approach in laboratory teaching, specifically a project-based teaching strategy. The objective is to encourage students to become more autonomous and to develop research initiatives in the lab work. The above-mentioned strategy has been applied to a senior school Physics Laboratory (“Técnicas experimentales IV” i.e., an Electrodynamics Laboratory). Since the laboratory is, as mentioned, implemented during the senior year, the students are supposed to be able to take control of their own learning process. They have also acquired some knowledge of the theoretical foundations of the topic, but some new concepts, not previously studied, will appear.
In this paper, two of the problems proposed to the students will be presented, and the result of that experience will be described. Both problems are intended to measure the propagation speed of light or an electromagnetic wave in general; the first in air (free propagation of a laser beam), and the later, inside a solid medium (guided propagation on the dielectric of a coaxial cable).

Two specific objectives can be remarked:
1. The value of the speed of light, at least in vacuum/air, is widely known, and even theoretically established. The first objective, then, is to get the student familiarized with problems of experimental design and uncertainty sources.
2. The students have been observed to assume that phenomena as interference or standing waves happen instantaneously: they tend to “forget” that the electromagnetic signals take time to propagate between two points, specially when dealing with propagation through transmission lines with lengths comparable to the wavelength.

The second of the proposed problems is, then, intended to make them realize this phenomenon and face its consequences.