Diffraction of X-rays and Laue Spots:
Rontgen discovered the X-rays, but he was unable to explain physical nature of it. A number of experiments were performed to test their nature by different scientists. It has been observed that light having wavelength of the order of 10-7 m can be diffracted by using an ordinary plane diffraction grating. We know that for diffraction, the space between any two successive lines on grating should be of the order of magnitude of wavelength; it is practically impossible to achieve diffraction of X-rays using grating.
According to scientist Max Von Laue, X-rays can undergo diffraction when passed through a thin plate of certain crystals and concluded that X-rays are electromagnetic waves. Optical grating provides diffraction in only one plane (i.e. acts as 2D grating). While in crystal grating the atomic centres of diffraction are not all in one plane but distributed in space i.e. crystal is thus 3D grating and hence the pattern of Laue spots observed on the photographic plate (as shown in the figure below) is found to be very complex.
But Professor Bragg gave a very simple interpretation of the diffraction pattern obtained by supposing that reflection occurs at various more populated atomic planes.