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My professor told us that for the sake of simplicity, we will assume that the order of reflection for Braggs' Law is always equal to 1 but failed to explain why. Is it because d(h,k,l) for materials has been determined using an order of reflection 1?

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The common form of the Bragg equation is $\lambda=d_{hkl}\sin(\theta)$ but in crystallography it is used as $\displaystyle \lambda =\frac{2d_{hkl}}{n}\sin(\theta)=2d_{nh,nk,nl}\sin(\theta)$ where the indices are divided through by $n$ to give $\lambda=d_{hkl}\sin(\theta)$. The indices $hkl$ have to be integers (from derivation via Laue equations) but can also be multiples of integers, i.e. they can have a common factor. When the $hkl$ do have a common factor they are not strictly lattice planes but planes parallel to $(h/n, k/n,l/n)$ with a spacing $1/n$ of the lattice. Suppose a plane is (210) then each lattice plane passes through a lattice point (atom). If the indices were (420) then not all planes are equivalent as only half of the points (atoms) pass through a lattice plane so they are not equivalent. Thus sticking with $n=1$ ensures we always use for example, (210), and not (420) or (630) etc., and so always calculate the correct value for $d_{hkl}$

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