# Laboratoire de Physique des Solides - UMR 8502

## Learning optics through experimenting

Teaching geometrical optics to first year university students is sometimes challenging, as this field of physics is often considered an old-fashioned and dusty science. Moreover, it is usually taught in a rather formal way, in which practical sessions are often a mere verification of the laws taught during preceding lectures. However, not only geometrical optics is particularly suited to be taught through experiments, but also it can serve as a useful introduction to experimental physics.

Over the past 10 years, we have developed at University Paris-Sud a first year course on geometrical optics centered on experiments made by the students. Interaction and self-questioning is strongly encouraged and practice comes first, before any theoretical knowledge.

Thus, the presentation of a phenomenon or concept (e.g. total internal reflection) requires three steps :

The students first discover the phenomenon through qualitative experiments that are mainly made by the students, or in some cases by the professor. For example, a laser is sent by the teacher in the direction of an opening at the bottom of a recipient filled with water. The water pours out of the recipient and forms a falling water stream in which light undergoes several successive internal reflections (see figure 1). Then, the students observe by themselves the successive reflections inside a Plexiglas ruler (see figure 2) and in an optical fiber.

The underlying physical concept (e.g. total internal reflection) is then presented by the teacher. For example, he/she presents the formal application of the law of refraction to total internal reflection. The existence and the expression of the critical angle for total internal reflection is derived.

The students go back to the experiment, and make quantitative measurements. For example, they measure the critical angle of total internal reflection using a Plexiglas hemi-cylinder (previously used for determining Snell’s law). From this measurement, they deduce the index of Plexiglas and its associated error.

Figure 1. Light is guided with total internal reflection in a falling water stream

Figure 2. Light is guided with successive total internal reflections in a Plexiglas ruler

This combination of experiments made by the students and presentation of theoretical concepts by the teacher takes place during so-called practical sessions, which take place in dedicated laboratories. There are also classical tutorial sessions that are alternated with the practical sessions.

As this course has existed for more than 10 years, its efficiency can be put into perspective. Feedback from the students is very positive : 90% of the students think that the organization of the teaching in practical sessions and tutorials is well adapted, and 70% of them find this course interesting.

### Reference

Learning through experimenting : an original way of teaching geometrical optics
C. Even, C. Balland and V. Guillet
European Journal of Physics 37, 065707 (2016)
doi:10.1088/0143-0807/37/6/065707
Article selected for the Highlights 2016 of European Journal of Physics.

Catherine Even