When light is projected onto a screen and an opaque body placed mid-way between the screen and the light source, an intricate shadow is cast on the screen made up of bright and dark regions quite unlike anything one might expect from geometrical optics. Frecesco Grimaldi in the 1600s was the first to publish a detailed study of this property of light, something he called "diffractio". The effect of diffraction is a general characteristic of wave phenomena occurring whenever a portion of a wavefront, be it sound, a matter wave, or light, is obstructed in some way.
Image - Diffraction pattern of a pinhole near the focus of a lens.
Diffraction fringes are evident in the shadows of laser light - which is a single frequency. The fringes would be filled in by multitudes or other fringes if the source were white light.
The physical property of diffraction will be explained in the following pages and it will be related to the XMM mission.