MATERIALS SCIENCE &ENGINEERING Anandh Subramaniam & Kantesh Balani Materials Science and Engineering (MSE) Indian Institute of Technology, Kanpur URL: home.iitk.ac.in/~anandh AN INTRODUCTORY E-BOOK Part of A Learner’s Guide Understanding Diffraction

 Diffraction sometimes seems a ‘mysterious’ phenomenon, which is difficult to understand.  We have noted that electromagnetic waves (light, X-rays etc.), water waves (i.e. elastic waves in a solid or fluid), matter waves (electrons, neutrons) etc. can be diffracted.  In fact it is best to start understading diffraction using water waves with a single slit.  Diffraction can be thought of as a special case of constructive and destructive interference (a case where there is a large number of scatterers*).  What are these scatterers? A: Any entity which impedes (partially and ‘redirects’) the path of wave can be conceived as a scatterer. Scatterers has to be understood in conjunction with the wave being considered [i.e. an entity may be a scatter for one kind of waves, but not for another (e.g. an array of atoms is a scatterer for X- rays, but it is not a scatterer for water waves)].  In a periodic array they can be entities of the motif [i.e. a geometrical entity (atoms, ions, blocks of wood), physical property (e.g. aligned spins) or a combination of both].motif  Experiments have been conducted where ‘matter waves’ have been diffracted from a crystal made of electromagnetic radiation (waves)! (Atoms diffracted from a Laser lattice).  We will use some ‘crude’ analogies and some ‘schematic cartoons’ to get a hang of this phenomenon → these should not be taken literally. Diffraction * Usually in a periodic array. ** Though other simple configurations may be envisaged.

 The bare minimum is one edge*. Two edges forming a single slit is better to get a better picture. What is the minimum I need to see diffraction/interference? * Which blocks part of the wave.

Let us start by throwing some balls on a wide slit. The balls in the gap pass ‘right through in a straight line’ (well most of the ones!), while the ones blocked by the obstruction reflect back (reflection not shown). Warning: these cartoons do not depict diffraction- they are a way to start visualizing the issues! 0 1 Screen ‘Intensity*’ on ‘screen’ Obstacle * Intensity ~ no. of balls/area/time Geometrical shadow region of zero intensity If we shine ‘incoherent light’ we will get a similar ‘intensity’ distribution. Near the edges the intensity will be different (but we will ignore this for now)

What about the ones hitting the edge? This is not what happens in diffraction. This is to tell you that ‘watch out for sharp corners’!! More cartoons on network 0 1 Screen Intensity on screen Obstacle Altered ‘intensity’ pattern (this is not one peak but a broad diffuse one as the way the balls hit the barrier edge will send them off in different angles) Centre of mass near edge. Glancing angle collision.

What if the slit width is of the order of the ‘ball size’? 0 Region of geometrical shadow There is ‘intensity’ in the ‘geometrical shadow’ region as well!  So we have seen that even with macroscopic balls it is possible to get ‘intensity’ in the region of the geometrical shadow.  For this effect to be prominent we have noticed that the slit width has to be of the order of the ‘size’ of the ball.

 Consider a series of speed breakers (bumps) on the road. Let a vehicle arrive at a velocity ‘v’. Another ‘crude’ analogy to understand diffraction

 In these set of slides we will try to visualize how constructive and destructive interference take place (using the Bragg’s view of diffraction as ‘reflection’ from a set of planes).  It is easy to ‘see’ as to how constructive interference takes place; however, it is not that easy to see how ‘rays’ of the Bragg angle ‘go missing’. Understanding constructive and destructive interference

Here we see waves scattered from two successive planes interfering constructively. (press page down button to see the successive graphics) Constructive Interference Note the phase difference of  introduced during the scattering by the atom.

Assuming that path difference of gives constructive interference: Similar to the path difference of, path difference of 2, 3 … n also constructively interfere. All Constructively interfere Also to be noted is the fact that if the path difference between Ray-1 and Ray-2 is then the path difference between Ray-1 and Ray-3 is 2 and Ray-1 and Ray-4 is 3 etc. Going across planes

Destructive Interference Exact destructive interference (between two planes, with path difference of /2) is easy to visualize. The angle is not Bragg’s angle (let us call it  d ).

At a different angle  ’ the waves scattered from two successive planes interfere (nearly) destructively Warning: this is a schematic Destructive Interference

 In the previous example considered  ’ was ‘far away’ (at a larger angular separation) from  (  Bragg ) and it was easy to see the (partial) destructive interference.  In other words for incidence angle of  d (couple of examples before) the phase difference of  is accrued just by traversing one ‘d’.  If the angle is just away from the Bragg angle (  Bragg ), then one will have to go deep into the crystal (many ‘d’) to find a plane (belonging to the same parallel set) which will scatter out of phase with this ray (phase difference of  ) and hence cause destructive interference.  In the example below we consider a path difference of /10 between the first and the second plane (hence, we will have to travel 5 planes into the crystal to get a path difference of /2).

 If such a plane (as mentioned in the page before) which scatters out of phase with a off Bragg angle ray is absent (due to finiteness of the crystal) then the ray will not be cancelled and diffraction would be observed just off Bragg angles too  line broadening! (i.e. the diffraction peak is not sharp like a  -peak in the intensity versus angle plot)  Line broadening can be used to calculate crystallite size (grain size).  This is one source of line broadening. Other sources include: residual strain, instrumental effects, stacking faults etc. Click hereClick here to know more about peak broadening. Click hereClick here to know more about peak broadening.