Solid state physicists prefer to write Bragg's law in a different form. For one thing, we talk about reciprocal lattice vectors, G, rather than about distances between lattice planes. The reciprocal lattice is a very powerful description of all lattice planes in a three dimensional crystal. The three vectors a*, b*, and c* form a basis of this reciprocal space. Orientation and distance between parallel planes are given by Miller indices, (h,k,l), which are the coordinates of the corresponding reciprocal lattice vector, G = ha*+ kb*+ lc*.
Bragg reflections are observed, if the scattering amplitude,
has a finite, non-vanishing value. Here, 
and 
are
the polarization vectors of the incident and outgoing beams, and
Q is the scattering vector, that is the
difference between the wave vectors of the outgoing and incident
beams, Q =
kout-kin.
Bragg's law, then, states that the scattering vector equals a
reciprocal lattice vector.
or, a little more explicitly,
The Miller indices of a reciprocal lattice vector are then used to identify the corresponding Bragg reflex.
