Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Diffraction patterns using sieves

Demonstration

A laser is used to illustrate how crystal geometry can be inferred from X-ray diffraction patterns.

Apparatus and materials

Laser, preferably mounted and mains powered

Sieve or fine mesh (nylon stocking)

Screen

Health & Safety and Technical notes


Check that the laser is labelled 'class 2'. 
 
Take care with reflections from the laser beam so that there is no danger of them striking a student's eye. 

Use sieves with the collar facing the laser.

Sieves are commonly used in geology and biology. You should use the 3 finest or mesh such as from a nylon stocking, stretched over a small embroidery frame. 
 
There should be only a low level of light in the room.

Procedure


a Start with laser beam showing a single spot on the screen. 

b Introduce the coarsest sieve and ask students to observe the pattern. 
 
c Remove and introduce finer mesh sieves showing that the pattern will broaden. 
 
d Using the finest sieve, move the sieve around but keep the same orientation. There will be no effect on pattern. 
 
e Twist the sieve to about 20 degrees from the normal to the beam. The beam expands in one direction. 
 
f Return to normal incidence. 
 
g Rotate the sieve. The pattern rotates. 

Teaching notes


Hopefully, the students will see that the diffraction pattern is a result of the angle of the mesh to the beam and the geometry of the mesh. If it doesn't work, the meshes available are too coarse. The same principles can be applied to X-ray crystallography or electron beam diffraction in metal films. 

This experiment was submitted by David Ferguson, the physics technician at Uppingham School