Fresnel and Fraunhofer Diffraction

Diffraction is classified into two distinct regimes, based on how far apart the source, the diffracting obstacle/aperture, and the observing screen are from one another.

FRAUNHOFER DIFFRACTION applies when these distances are effectively very large (in practice, achieved using lenses rather than genuinely enormous physical distances) -- the incident wavefront reaching the obstacle is essentially a PLANE wavefront, and the diffracted light is likewise observed at effectively infinite distance (again achieved with a lens). This is the mathematically simpler case, and the one this chapter analyses in full for a single slit (Section 7.9.3). …