Diffraction

Diffraction is the spreading and patterning of waves as they pass through openings or around obstacles. It is strongest when those features are comparable in size to the wavelength.

Huygens–Fresnel principle: A wavefront can be modeled as secondary wavelets whose interference determines the wave's subsequent form. It provides a construction for predicting how waves spread beyond an opening or obstacle.
Wavelength: The spatial period of a wave, measured as the distance between successive points in the same phase. It sets the scale against which an opening or obstacle must be compared.
X-ray crystallography: A method that infers crystal structure from the diffraction of X-rays by regularly spaced atoms. Crystal diffraction patterns reveal atomic arrangements through their spacing and intensities.
Geometrical optics: An approximation that models light as rays traveling along paths governed by reflection and refraction. It neglects wave spreading and interference, which become essential near apertures and edges.
Single-slit diffraction: The diffraction pattern produced when a wave passes through one narrow slit. Its bright and dark fringes show how contributions from different parts of an opening interfere.
Superposition principle: The principle that overlapping waves combine by adding their displacements or field amplitudes. Diffraction patterns result from the superposition of waves arriving along different paths.
Bragg's law: The equation relating X-ray wavelength, crystal-plane spacing, and angles of constructive reflection. It translates diffraction angles into distances between crystal planes.
Scattering: The redirection of waves or particles after interaction with matter or an inhomogeneity. Scattering is a broader interaction; diffraction describes a characteristic wave pattern from constrained propagation.
Fraunhofer diffraction: The far-field regime of diffraction, in which rays reaching the observation plane are effectively parallel. This approximation yields compact formulas for patterns observed far from an aperture.
Interference: The redistribution of wave intensity caused when waves combine constructively or destructively. Alternating reinforcement and cancellation create diffraction fringes.

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