Secondary waves of light

Abstract

It has hitherto been held that, so long as the diffraction apertures used (cut in perfectly opaque or perfectly reflecting screens) are large compared with the wave-length of light, Fresnel's expression for the amplitude of the disturbance due to a surface-element gives us a close approximation to the observed diffraction effects, and that the exact value for the obliquity factor is of little importance (e.g. see Schuster's "Optics", sec. 48). That this is true only in the special case in which the apertures are held normal to the, waves of light, and not in other cases, is shown by some new diffraction phenomena that I have made the subject of study.