Page "X-ray crystallography" Paragraph 16

Here d is the spacing between diffracting planes, is the incident angle, n is any integer, and λ is the wavelength of the beam.

These specific directions appear as spots on the diffraction pattern called reflections.

Thus, X-ray diffraction results from an electromagnetic wave ( the X-ray ) impinging on a regular array of scatterers ( the repeating arrangement of atoms within the crystal ).