# Introduction

The concept of reciprocally has been introduced in the Diffraction and Imaging TLP, and in the X-ray Diffraction TLP within the Bragg and Scherrer equations. This inverse scaling between real and reciprocal space is based on Fourier transforms.

Josiah Willard Gibbs first made the formalisation of reciprocal lattice vectors in 1881. The reciprocal vectors lie in “reciprocal space”, an imaginary space where planes of atoms are represented by reciprocal points, and all lengths are the inverse of their length in real space.

In 1913, P. P. Ewald demonstrated the use of the Ewald sphere together with the reciprocal lattice to understand diffraction. It geometrically represents the conditions in reciprocal space where the Bragg equation is satisfied.

Later in the TLP, we will formalise the relationship between the real lattice vectors and unit cell and the reciprocal lattice, show the construction of the Ewald sphere and demonstrate some if its uses.