1. The single slit interference pattern and the double slit interference pattern that are observed are actually due to diffraction as well as interference. The bottom pattern and the pattern to the right are called Fraunhofer Diffraction patterns. Fraunhofer diffraction for a circular aperture.  a Secondary wave sources from an aperture. Destructive interference occurs when  = /2. We can determine the location of the dark fringes for a single slit using: This expression is from geometry, and is similar to the expressions obtained for Young’s double slit experiment.