Fig Interference diagrams for N equally spaced very narrow slits. (a) N = 2 slits (b) N = 8 slits (c) N = 16slits

Fig A portion of a transmission diffraction grating. d sin  = m m = 0,  1,  2, … at intensity maxima

Fig A diffraction-grating spectrometer. The lenses between source and grating form a beam of parallel rays incident on the grating. The beam is diffracted into various orders satisfying d sin  = m  where  m = 0,  1,  2, …

Fig X-ray diffraction by a crystal.

Fig Model of the arrangement of the ions in a crystal of sodium chloride. Na in black, Cl in red. The spacing of adjacent atoms is nm.

Fig (a) Scattering of waves from a rectangular array. (b) Constructive interference occurs for waves scattered at angles such that a cos  a = a cos  r. (c) Interference from adjacent rows is also constructive when path difference is 2d sin  = m. (Note  =  a =  r)