The X-rays penetrate deep in the crystal: W.L. Bragg found the diffraction to be a consequence of the simultaneous reflection of the X-ray beams by various lattice planes belonging to the same family.

In the Figure on the left , is the angle between the primary beam (direction given by the unit vector s₀ ) and the family of lattice planes (hkl) ( with no integer common factor larger than unity among the indices).

The difference in path between the waves scattered in D and in B (in the direction s ) is equal to

The maximum positive interference will occur when:

which is Bragg's equation. The angle for which (1) is verified is the Bragg angle _B. Owing to the large number of planes involved in the reflection, the reflected waves will interfere destructively if (1) is not verified.

More values for _B can satisfy (1) for fixed values of and d_H .

It is then possible to speak, for the same family of lattice planes (hkl), of diffraction effects of first order, of second order, etc. according to the value of n.

The point of view can be semplified by writing (1) in the form:

If we consider the family of fictious lattice planes (h' = n h, k' = n k, l' = n l), we can replace into (2) d_H /n  by d_H'   so that (2)  becomes: