Historical review and introduction to biological applications

The wave nature of light on which diffraction phenomena are based was first suggested by Huygens more than 300 years ago. About 100 years later, Haüy wrote an essay on the regularity of crystal forms that is considered to be the beginning of crystallography.

1690

In his Treatise on LightC. Huygens wrote that light ‘spreads by spherical waves, like the movement of Sound’, and explained reflection and refraction by wave constructions.

1784

R.-J. Haüy a mineralogist, published his theory on crystal structure, following observations that calcite cleaved along straight planes meeting at constant angles.

1895

J. J. Thomson discovered electrons during an investigation of cathode rays. He initially called them corpuscles.

1895

W. C. Röntgen discovered X-rays. While experimenting with electric current flow in a partially evacuated glass tube, he noted that a radiation was emitted that affected photographic plates and caused a fluorescent substance across the room to emit light.

1912

P. P. Ewald's doctoral thesis on the passage of light waves through a crystal of scattering atoms led M. von Laue to ask what would happen if the wavelength of the light were similar to the atomic spacing, and this led to the first observations of X-ray crystal diffraction by W. Friedrich, P. Knipping and von Laue. Because of their short wavelengths, X-rays provide a ‘ruler’ with which to measure distances between atoms.

1912–15

W. H. Bragg and W. L. Bragg interpreted diffraction in terms of reflection from crystal planes.