We typically use the word "interference" to describe the superposition and interaction of a few waves as in Young's Double Slit Experiment.
We typically use the word "diffraction" to describe the superposition and interaction of many waves as in the Diffraction Grating or the diffraction effects of a single slit or the diffraction effects of a circular opening or the diffraction effects of light as it goes around any object.
We will look at the diffraction of a single slit.
In the straight forward direction, light from all parts of the slit travels the same distance and arrives "in phase" so there is a bright central maximum.
A single slit diffraction pattern has a bright central maximum surrounded by much smaller maxima.
Return to Ch 25, Wave Optics: Interference and Diffraction (c) Doug Davis, 2002; all rights reserved