Usage of microscope slides is a variation on this concept. Instead of a convex lens over a flat lens, there are two lenses at an angle to each other. This can be achieved by placing a piece of hair or a piece of paper at the edge of the two slides. This will create an air wedge between the two lenses. The experiment can be used to approximate the angles created by the slides' imperfections. This allows for the angle to be found, using the wavelength of light, and the distance between fringes.