Schematic diagram for Bragg diffraction of X-rays in a solid state regular crystal, the black balls representing atoms of the crystal. Only X-rays impinging from the left that on the right have experienced a travel delay of 1, 2 or any whole number of wave lengths, will coherently come together and reinforce each other. The condition of this constructive interference imposes the Bragg relation

${\displaystyle 2d\sin \theta =n\lambda \,}$