Bragg’s law gives the relation between the spacing of two atomic planes and the angle of incidence at which these planes produce maximum diffraction. For maximum intensity, any wave incident on such planes should be scattered in such a way that they constructively interfere. Bragg’s law gives the condition for constructive interference and is given by,

Where 'd' is the spacing between the two atomic planes, 'θ' is the angle of incidence or the angle of diffraction (Bragg's angle), 'n' is the order of diffraction and 'λ' is the wavelength of the incident wave.

Consider two successive layers of atoms or the atomic plane. When two waves of wavelength λ are incident on the atoms with an atomic spacing of d between them, the waves are scattered by the atoms. These scattered waves interfere among themselves either constructively or destructively. The constructive interference happens only when the path difference between the two waves are equal, i.e. they are in phase. This happens when the path difference between the waves approaching a point is an integral multiple of the wavelength, λ and the angle of incidence equals the angle of scattering.

The first ray gets scattered by particle A and travels along AC’. There will be a path difference between the ray which travels along AC’ and the one which transmits to B and then gets scattered by B and travels through BC. The path difference is represented by CC’. This path difference has to be an integral multiple of the wavelength for constructive interference to occur.

Path difference is given by,

From the figure, from triangle ABC,

Also from the figure,

So the path difference can be calculated as,

So the condition for constructive interference can be written as,

This equation is known as Bragg’s law and gives the condition for the constructive interference of the waves scattered by two atomic planes. The angle at which this takes place is known as Bragg’s angle or the diffraction angle.