Fizeau fringes at home

We often come across Fizeau fringes outside the optical laboratory, for example, in an oil film floating on a wet street illuminated by sunlight, or on a soap bubble. In both cases beautiful interference color patterns are seen when a white light source is involved. Some of these patterns are the phenomena we normally mention to students to focus their attention on occurrences of equal-thickness interference in their daily environment. In this paper we present a simple experiment that students can do either in their own homes or in a computer laboratory to take the study of such phenomena one step further. The main advantages of this experiment are the controlled conditions, in that the visibility of each fringe pattern can be modified in real time with the scanner control software and that the fringes can be recorded for later discussion and analysis. We have divided the experiment into two clearly different steps. In Sec. II we describe how to obtain stable Fizeau fringes with a flatbed scanner, which the students can easily do on their own. Subsequently in Sec. III we discuss the background that students should receive to understand the observed interference. This background information is not always available in a basic optics laboratory because of a lack of instruments, such as a spectroradiometer, for example, so we present the essential data needed to stimulate the hoped-for discussion. The proposed experiment should be done after having discussed equal-thickness interference fringes ~also known as Fizeau fringes, or sometimes contour fringes! and the Newton’s ring experiment. We recommend that students should also have some knowledge of partial optical coherence. A basic grasp of colorimetry, while not essential, might help deepen the discussion of the topic. If the students have a background in multiple interferences alone, the proposed experiment will give them a practical application of their theoretical knowledge.