A method is described by which a multiple‐slit interference pattern can be demonstrated quite easily with the help of an inexpensive Fabry–Perot etalon and a low‐power laser beam. The separation between the ’’slits’’ can be adjusted continuously. The experiment brings forth the similarity between a grating and a Fabry–Perot as a fringe‐sharpening device. A simple derivation of the analytical expression for such fringes is presented.