in an XRD pattern graph, we have 2 theta in the x-axis...why not just theta??
It is partly historical and has become convention. Older diffractometers were constructed using theta-2theta geometry. The tube remains stationary while the sample rotates on the theta circle and the detector rotates on the 2-theta circle, thus maintaining the equivalence of incident and diffracted angles. So in this case the signal at the detector is measured in degrees 2-theta. It is becoming more usual to use theta-theta geometry for powder diffractometers where the sample remains horizontal (or vertical in transmission) and both the tube and the detector rotate on theta circles. The incident angle from the tube to the sample is theta degrees and the diffracted angle from the sample to the detector is also theta degrees; with respect to the incident beam the signal at the detector is the sum, or 2-theta degrees. The resulting x-axis on the diffractogram is labelled as 2-theta.
theta is the incident angle and the beam is deflected by an angle 2 theta
theta is the angle between incident ray and crystallographic plane
2 theta is the angle between reflected beam and transmitted beam
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