The position of diffracted beams Braggs law

A beam of radiation will only be diffracted when it impinges upon a set of planes in a crystal, defined by the Miller indices (hkl), if the geometry of the situation fulfils quite specific conditions, defined by Bragg s law  

2Note that it is crystallographic convention to use the Angstrom unit, A, as the unit of length. The conversion factor is 1A = 10-10 m 1 nm = 10 A. Here the SI unit, (nm), will be mainly used, but A may be employed from time to time, to avoid unnecessary confusion with crystallographic literature. 

Equation 6.1a includes an integer n, which is the order of the diffracted beam. Crystallogra-phers take account of the different orders of diffraction by changing the values of (hkl) to (nh nk nl). Thus the first order reflection from, for example, (111) planes, occurs at an angle given by  

The second order reflection from the same set of planes then occurs at an angle  

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