Diffraction theory is presented for students who want to learn“black box”of the protein crystallography. The description that“the crystal works as a three dimensional diffraction lattice”is fully explained based on the wave nature of X-ray and the periodic nature of crystal. The reciprocal lattice is introduced as a means for calculating the lattice plane spacings. Using the reciprocal lattice, the diffraction condition is derived again with the concept of reflection sphere. The structure factor and the Fourier method are also introduced in the context of obtaining the electron density of the crystal.
