Symmetrically spaced atoms cause re-radiated X-rays to reinforce each other in the specific directions where their path-length difference, 2d sin θ, equals an integer multiple of the wavelength λ (Photo credit: Wikipedia)
X-ray crystallography. X-rays are unique in that their wavelength is about the length of bonds within molecules. When X-rays hit a crystallized molecule, the electrons surrounding each atom cause the beam to bend. Because there are many atoms the result is that when the X-rays exit the crystal and fall onto a photographic plate, they produce a series of light and dark patches. Measuring the intensity and relative position of each patch indicates the relative positions of atoms within the crystal.
With her co-worker John Desmond Bernal (1901-1971) Hodgkin produced the first diffraction patterns for proteins.
‘Description of the basic helical structure of the DNA molecule’
Her work is used, unaccredited, in Watson & Crick’s Nobel Prize-winning paper, from information ‘secretly’ leaked from Maurice Hugh Frederick Wilkins.
Through her work on X-ray diffraction, she realised that the ‘backbone’ of the DNA molecule was on the outside.
By 1952, Franklin had taken the clearest pictures of the molecules to date, which provided evidence of a helical, or spiral structure.
Watson & Crick would eventually articulate a ‘double-helix’ construction.