Experiment with diffraction

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low-Cristales-IMG_0287 This interactive installation gives you the chance to experiment with the principal tool of crystallography: diffraction. The two lasers and fourteen diffraction lattices available will allow you to explore many of the fundamental concepts of diffraction, as important for crystallography as they are difficult to understand – sometimes counterintuitive.

Before using this installation you should familiarise yourself with the material presented in the panels “Diffraction”, “Crystals and X-Rays: Minerals and Medicines” and “Crystals and X-Rays: Biological Macromolecules”.

Position the slides on this table in front of the laser beam and observe the diffraction pattern that each periodic pattern produces. Change the beam between the different samples of each slide and see how the diffraction pattern changes. Be careful not to expose your eyes to the direct beam of the laser!

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The panel photographs show the patterns printed on each slide, so you can see the real and the diffraction patterns at the same time, as well as the laser beam, which has a wavelength of 340nm (green) or 632nm (red). Crystallographers work with X-rays to be able to see molecules by means of diffraction experiments. X-rays, with a length around 0.1nm, are not visible, and neither are the molecular patterns in the crystal. This is precisely the usefulness of the crystallographic methods of X-ray diffraction: to “see” the molecules in the crystal.

The experiments you can perform at this table to see the fundamental concepts in diffraction are:

 

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Reciprocity

The diffraction pattern presents a reciprocal relation with the real pattern. The greater the distance between the dots on the slide, the less the separation of the beams diffracted and therefore of the dots on the screen.

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Unit cell

The periodicity of the diffracted beams depends on the unit cell. A spun rectangular cell (A) produces a diffraction pattern with spun rectangular cells, and the same thing happens with a pattern with a hexagonal cell (B).

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Motif

The motif that is repeated in the real pattern controls the intensities of the diffracted beams but not their position. Test it out using these two patterns with the same distribution but different motifs.

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Symmetry

The symmetry of the real pattern controls the symmetry of the diffracted pattern. The planes of vertical symmetry in patterns A and B are the same, but the planes of horizontal symmetry are only present in A.

The elements of symmetry make certain diffraction beams disappear from the diffracted pattern. This effect is called “Systematic Extinction” and enables the crystallographer to identify the symmetry of the crystal. Can you see which beams are “extinguished” in C and D

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DNA

With this slide you can investigate the famous diffraction pattern of DNA of Rosalind Franklin. The parts of the helix that curve to each side produce the two crosses/X’s of the diffracted pattern. Test it out using A and B. With C you can observe the complete pattern of the helix.

The presence of the second DNA helix adds details to the diffraction pattern. These “details” are the information that makes it possible to determine the structure of the real pattern out of the diffraction pattern.

 

CRISTALES

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