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Why are we taking 2$\theta$ instead of $\theta$ in X-ray powder diffraction (XRD). I have found the forum post 2 theta in X-ray Powder Diffraction (XRD), but there is no answer. What is the explanation?

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For the $\theta : 2\theta$ goniometer, the X-ray tube is stationary, the sample moves by the angle $\theta$ and the detector simultaneously moves by the angle $2\theta$. At high values of $\theta$, small or loosely packed samples may have a tendency to fall off the sample holder.

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When you do diffraction, $\theta$ is the angle of incoming EM wave, as well as the angle of difracted EM in regard to Bragg's planes. So the total change in angle of the EM wave equals $2\theta$.

See images at http://en.wikipedia.org/wiki/Bragg%27s_law.

If this is not the answer you're looking for, maybe you should specify your question more clearly. (I was doing Bragg's experiments as a student decades ago, I naturally always halfed the angle, so I do not quite understand what is your problem.)

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