Father Sir William Henry Bragg and Son Sir William Lawrence Bragg proposed that Bragg's diffraction occurs when electromagnetic radiation or subatomic particle wave with a wavelength comparable to atomic spacing is incident upon a crystalline sample and scattered in a specular fashion by the atoms in the system and undergoes a constructive interference.

For a crystalline solid, the waves are scattered from lattice planes separated by the inter-planar distance d. When the scattered waves interfere constructively, they remain in phase (demonstrated later) since the path length of each wave is equal to an integer multiple of the wavelength.

The path difference between the two waves undergoing constructive interference is given by 2d sinθ, where θ is the scattering angle. This leads to Bragg's law, which describes the condition for constructive interference from successive crystallographic planes (hkl ) of the crystalline lattice.