A diffraction experiment at the near field: the homemade Talbot effect

Abstract

Diffraction refers to a kind of optical phenomena which occurs when light approaches an element (object or aperture) whose features are in the range of the illuminating wavelength (small apertures, sharp edges). It can be explained by means of the undulatory nature of light or also geometrically by using simple ray optics. Diffraction phenomena are impressive and not intuitive, so it makes them very interesting to bring examples to the classroom. The most popular diffraction experiments show effects in Fraunhofer regime, that is to be said, far from the diffractive object. Common examples are the single or double slit experiments. In this manuscript, we propose and show a less common diffractive effect that occurs in the Fresnel regime, near to the diffractive object. It is the Talbot effect or self-imaging phenomenon, which appears by illuminating a diffraction grating with a collimated monochromatic beam. It consists of the apparition of replicas (self-images) of the grating intensity pattern at periodic distances, multiples of the so-called Talbot distance. We show how this effect may be shown into the classroom with cheap and easy to find elements. In addition, we take advantage of its dependence on the coherence degree of the source to introduce the concept of optical coherence and show its effect on the contrast of the Talbot self-images. These experiments could be appropriate for undergraduate students or introductory physics courses.