Braggette

Meet Braggette, an X-ray who only shines back brightly from a crystal at just the right angle.

A crystal is a regular stack of atomic planes a distance d apart. X-rays reflecting from neighbouring planes travel different distances, with the deeper ray going an extra 2d sin θ. When that extra path is a whole number n of wavelengths, the reflected waves line up and add into a strong beam. At other angles they largely cancel. This is Bragg's law, 2d sin θ = nλ.

William Henry Bragg and his son William Lawrence Bragg worked this out in 1913 and used it to work out crystal structures. They shared the 1915 Nobel Prize in Physics. Example: copper Kα X-rays (λ ≈ 0.154 nm) reflecting from planes 0.2 nm apart give a first-order peak at sin θ ≈ 0.385, θ ≈ 23°.

X-ray diffraction revealed the structure of salt, metals and minerals, and of biological molecules: Rosalind Franklin's X-ray images were key evidence for the double helix of DNA. It is still a standard tool in materials science and drug research.

Real crystals have defects and thermal motion, which broaden and weaken the peaks.