This applet models Bragg's Law of Diffraction, which is of crucial importance to the analytical technique of XRD (X-ray diffraction).
This technique allows the accurate determination of interatomic distances in crystals. In other words, if you can make a compound into a crystal, you can use XRD to determine the structure. It was Linus Pauling's use of this technique which determined the two secondary structures of proteins called the α-helix, and β-pleated sheet.
Click on dist+ to increase the distance between planes of the crystal (in blue), and on dist- to decrease the distance. Click on theta+ to increase the incident angle θ of the X-ray radiation, and on theta- to decrease it. Clicking on lambda+ increase the wavelength λ of the X-ray radiation, and on lambda- decreases it. The relevant values of each parameter are displayed on the right side of the applet. The angle θ is measured from the normal to the planes of the crystal.
The crucial idea of this technique is as follows: each part of the incoming X-ray beam (on the left in red and green), is in phase, so there is constructive interference. When the beam is diffracted by the crystal, the resulting parts of the beam on the right are only in phase for certain combinations of dist, θ, and λ. The condition for constructive interference is n λ = 2 dist sin θ (called Bragg's Law), where n is an integer. In the applet, we know wwe have constructive interference when the red and green waves on the right side of the crystal are in phase with each other.
How can we use this law to determine structure? We shine a beam of X-ray radiation on our sample, and slowly rotate the crystal. We notice at which angle θ the amount of diffracted light is a maximum. Since we know the wavelength λ of our source, we plug into the Bragg equation, and we calculate the distance (dist) between the planes of the crystal.
The discontinuity (break) in the wavy lines (X-ray beams) at the point of diffraction is an artifact of the way the process is depicted here.
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