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Bragg's law

1.33 λ path difference · I = 6% of maximum

Scattering geometry

Reflected waves

Intensity against angleclick to set θ

Bragg's law

X-rays reflect weakly from each plane of atoms in a crystal. The beam reflected from one plane down travels an extra distance 2d sin θ, so all the reflections add up in phase only when .

The intensity plot shows the interference of N planes, with . A real crystal has millions of planes, which is why diffraction peaks are so sharp.