Bragg's model of X ray diffraction :

According to Bragg's, a crystal which is made up of series of equally spaced atomic planes, can be treated as a transmission grating and a reflection grating as well. When X-rays are incident on a crystal face, some of the rays will pass undeflected and some other rays will penetrate into the crystal and strick the atoms in successive planes. From each of these plans the X-rays are reflected (i.e., X-rays strick with electrons in the atoms of the structural units and undergo a change in direction), in all directions. For the formation of an intense diffraction pattern in any direction the condition is that the angle of incidence should be equal the angle of reflection of the beam for the direction studied. 

             Bragg's equation can be applied either to the reflection or the diffraction experiment. In the case of reflection from the crystal surface, the angle ∅ stands for the angle between the crystal surface and the duration of incident X-rays. In the case of diffraction of X-rays on passing through the crystal ∅ is one half of the angle between the diffracted beam and the direction of the undeflected incident beam. The angle 2∅ is called the diffraction angle. Thus, the inter planer distance, d may be found either by the reflector or the diffraction of X-rays.