ABSTRACT
First identified by Francesco Maria Grimaldi (1618-1663), diffraction refers to nonrectilinear deviations of light passing by, around, or through openings in opaque objects that cannot be ascribed to reflection or refraction. The phenomenon was initially observed as a small widening of the shadow and penumbra of a narrow object, accompanied by bands or fringes of light and color both outside and (more weakly) inside the edges of the shadow. Grimaldi called this diffractio in accord with the view-set forth and supported by these and other experiments in his Physico-mathesis de lumine (1665)—that light is a rapidly moving undulating fluid that is “split apart” and its undulations altered when flowing past the edges of corporeal objects.
