Editing Fraunhofer diffraction (section)

===Diffraction by a rectangular aperture===

[[File:Rectangular diffraction.jpg|right|200px|thumbnail|Computer simulation of Fraunhofer diffraction by a rectangular aperture]]The form of the diffraction pattern given by a rectangular aperture is shown in the figure on the right (or above, in tablet format).<ref>{{harvnb|Born|Wolf|1999|loc=Figure 8.10}}</ref> There is a central semi-rectangular peak, with a series of horizontal and vertical fringes.  The dimensions of the central band are related to the dimensions of the slit by the same relationship as for a single slit so that the larger dimension in the diffracted image corresponds to the smaller dimension in the slit.  The spacing of the fringes is also inversely proportional to the slit dimension.

If the illuminating beam does not illuminate the whole vertical length of the slit, the spacing of the vertical fringes is determined by the dimensions of the illuminating beam.  Close examination of the double-slit diffraction pattern below shows that there are very fine horizontal diffraction fringes above and below the main spot, as well as the more obvious horizontal fringes.