We require waves to explore the structure of crystals. Such waves are able to interact with ats. For this purpose its wavelength must be comparable with the interayomic spacing in crystals i.e. of the order of 1 angestrom.X ray is the most convenient wave suitable for the study of crystals. Xrays can be diffracted by crystals just in the same way as the visible light is diffracted by a diffraction grating .Crystals can be used as diffraction grating for the diffraction of xrays.When monochromatic beam of Xrays incidents on the atom ,harmonic vibrations of electrons begin to start. Frequency of such vibrations is equal to the frequency of the incident wave.. Due to these vibrations electrons accelerated .These accelerated electrons re-emit the radiation at the frequency of their vibration ( frequency of incident radiation)
We are concerned with a group of atoms arranged in a regular pattern in crystals. Let a row of identical atoms are cosidered. There falls a plane xray wave normally to this group of atoms. Each atom of the row emits the radation.
The incident wave crests is assumed to fall parallel to the row of atoms. The envelope of the wavelets emitted by individual atoms forms new wave crests and we see that besides a beam propagated in the same direction as the incident beam, there are beams in few other specific directions also. Thus althoughthe individual atoms re-emit radiation in all directions, there are only a few directions in which these wavelets reinforce eachother to produce plane waves, These waves are said to be produced by diffraction and are named as zeri order, first order, second order diffracted beams. The problem of a row of atoms was considered only for simplcity. Actual crystals have three dimensional character. There are two ways of calculating the conditions of diffraction in actual crystals, that due to Bragg and that due to Von Lue .
Ref:-books by RL Singhal , C.Kittel, Gupta-kumar!
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