J. C. KENDREW and M. F. PERUTZ 



chromatic x-ray beam, successive diffracted rays will ' flash out ' when 

 the angle of incidence becomes appropriate for each plane in turn ; the 

 various types of x-ray camera are designed to photograph these rays 

 and their geometry is arranged so that each ray can readily be identified 

 with the set of planes which caused it. This kind of diffraction is 

 rather like optical reflexion from a set of parallel glass plates, and it 

 is usually referred to as x-ray ' reflexion ', and the diffracted rays 

 from a set of lattice planes are simply called ' reflexions '. 



Figure 3. Space lattice showing lattice 



planes in different orientations. 



{Reproduced from Bunn*.) 



Just as the diffraction angle of the rays from a grating can be used 

 to deduce the periodicity of its rulings, so the diffraction angle of the 

 x-ray reflexions can be used to deduce the distance between the planes 

 in the crystal (the spacing d in the above equation), and hence to find 

 out the dimensions of the unit cell — always the first step in the x-ray 

 analysis of a crystal. 



Returning to the optical grating, we note that while the diffraction 

 angle is a function of the periodicity of the rulings (the scattering units), 

 their relative intensities are governed by the shape of the rulings. This 

 shape will depend on the tool used to engrave the grating. Indeed the 

 diffracted rays will differ not only in intensity (or amplitude) but also 

 in phase, though differences in the latter are of course not apparent 

 to the eye, and both amplitude and phase will be functions of the 

 shape of the rulings. 



If a beam of x-rays falls on matter the electrons contained in the 

 matter are responsible for the scattering which takes place, and so the 



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