A Student Experiment About Rainbows: Ray Tracing Through an Acrylic Cylinder

Nearly everyone, including physics students, finds rainbows to be fascinating and much has been written about them. For example, in a 2020 paper, Kenneth Ford sets forth the basic theory of rainbows created by water droplets at the level of geometric optics and uses a graphical approach to address the question of the relative intensities of the light in the arc of the rainbow and below the arc. However, little has been written about related experiments that students can carry out. In this paper we describe an experiment easily accessible to students that clearly illustrates the principles of rainbow formation. Since experimenting with spheres of either water or other transparent material is difficult, the experiment is carried out with cylinders. The principles are the same. Ivanov and Nikolov have used cylindrical bodies to demonstrate rainbows, but without reference to student activities.