Thin film interference: an experiment with microwaves and paraffin oil

D'Anna, Michele and Corridoni, Tommaso (2015) Thin film interference: an experiment with microwaves and paraffin oil. The Physic teacher (53). pp. 475-481. ISSN 0031-921X

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Thin film interference manifests itself in a wide range of visually pleasing situations in everyday life (in the coloured effects caused by a drop of oil on water, in soap bubbles, etc.) and is also involved in important technical applications (semi-reflecting mirrors, anti-reflection lenses, etc.). Yet, despite its familiarity, high school students are rarely asked to consider this common phenomenon, in particular from an experimental point of view. In this paper we present an experiment to test quantitatively the thin film interference relation. Since experiments with visible light involve layer thicknesses too small to be managed in a high school laboratory, we use microwaves of wavelength l = 2.85 cm. Furthermore, we choose to work with paraffin oil1, so that we can easily change the thickness of the layer. As expected, the measured intensity of the transmitted wave shows a modulation with the thickness of the layer, so that an accurate data analysis enables us to determine the refractive index of the paraffin oil in the chosen wavelength range. The value obtained also corresponds nicely with the measurements made using a Michelson & Morley interferometer. From a didactical point of view, this experiment can be considered as a quantitative alternative to those commercially available, showing interference fringes with visible light and thin wedge of air. The advantages are: 1) by changing the thickness themselves, students discover step by step the modulation of the intensity of the transmitted signal; 2) the expression thin film emerges immediately as a concept that correlates the thickness of the layer with the wavelength we are considering; 3) the experiment can also be seen as an introduction to relevant experimental techniques, such as time resolved reflectivity2, or to interference experiments at low intensity, as those illustrated by R.P. Feynman in the first chapter of his QED3.

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