Diffraction and Crystal Structure

Diffraction is usually considered to be a phenomenon associated with waves. Waves are known to have properties of reflection, refraction, interference and diffraction. Diffraction of waves is defined by bending of waves around an obstacle present in path of waves. A famous experiment known as Young's double slit experiment is considered to be the first instance that made diffraction prominent in the study of waves.

Schematic of double slit experiment for study of diffraction

 After observing the interference pattern obtained due to diffraction of light, diffraction have of waves have been used in many applications. Many devices or instruments have been developed that are used to study diffraction. Most widely used application of waves diffraction is to determine crystal structure with the help of X-rays. Inter atomic and inter planar space in crystals is almost of same size as that of wave length of X-rays. As condition for diffraction is that wave length of waves should be comparable to the size of slit which in case of crystals is inter atomic space so X rays are good candidate to study crystal structure via diffraction. This study of crystal structure is governed by Bragg's law. Bragg is the name of scientist who first successfully explained the diffraction of waves from crystal. If we know wave length of incident wave and glancing angle (Angle with which a wave is incident on crystal), then we can easily determine inter planar spacing. Instrument used for determining crystal structure with the help of Bragg's law is known as Bragg's spectrometer. Bragg's spectrometer measures the intensities of waves after passing through crystals and determine whether these waves are interfering constructively or destructively. When diffracted interfering waves interfere constructively, a bright spot is observed and when these interfere destructively a dark spot is observed. There are other methods also available that make use of diffraction for structure determination of crystals. These methods include powder method, electron diffraction, neutron diffraction, Laue's method etc. Mentioning of electron and neutron diffraction must be shocking to some readers but this is another reason that make diffraction an ever green phenomenon to be explored and studied. Louis de Broglie, in 1924, suggested that matter has dual nature. This means that matter can also behave as wave under certain conditions. By using momentum and Planck's constant , one can easily determine wave length of wave associated with a particle. This wavelength is equal to the ratio of Planck's constant and momentum of particle. This wavelength for massive objects is very small and can not be detected with present day equipment however for small particles like electrons and neutron it has appreciable wavelength and can be measured and observed. This wave nature of particles has now been verified and is also being used for studying crystal structure. Wave nature of electrons was also verified by using double slit experiment. Presence of interference pattern on observation screen proved that electrons behaved as waves and produced interference pattern due to diffraction from slits. Sill large scale research is being carried out for the improvement of experimental setups involved in crystal structure studies. In schools and colleges an instrument named diffraction grating is used for the demonstration of diffraction of light. Here you can see diffraction, its physical significance, and application in a small video. click to watch. 

If you are curious enough to observe interference by yourself, then the good news is that you can do it with great ease at your home and all you need is a torch and solid sphere ( a tennis ball ). Get inside a room, turn off lights, turn your torch on and point towards the wall. Now place the ball in the path of torch light. You will observe a bright spot at the center of shadow of ball. The bright spot in due to diffraction (bending of light around obstacle). If diffraction had not occurred , we would not be able to observe dark spot. Do this fun activity and share your results with us about diffraction of light in comment section.