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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Diffraction | 1/5 | https://en.wikipedia.org/wiki/Diffraction | reference | science, encyclopedia | 2026-05-05T10:54:57.453682+00:00 | kb-cron |
Diffraction is the deviation of waves from straight-line propagation due to an obstacle or through an aperture, without any change in their energy. Diffraction is the same physical effect as interference, but interference is typically used for the superposition of a few waves, while the term diffraction is used when many waves are superposed. The term diffraction pattern is used to refer to an image or map of the different directions of the waves after they have been diffracted. Diffraction patterns are pronounced when a wave from a coherent source (such as a laser) encounters a slit/aperture as shown in the first image. In classical physics, diffraction is described by the Huygens–Fresnel principle that treats each point in a propagating wavefront as a collection of individual spherical wavelets. The patterns are due to the summation over different points on the wavefront (or, equivalently, each wavelet) that travel by paths of different lengths to the registering surface. If there are multiple closely spaced openings, a complex pattern of varying intensity can result. Other types of apertures or obstacles lead to different patterns, some of which are described later on this page. These effects occur when a light wave travels through a medium with a varying refractive index, or when a sound wave travels through a medium with varying acoustic impedance – all waves diffract, including gravitational waves, water waves, and other electromagnetic waves such as X-rays, radio waves as well as matter waves such as electrons and neutrons. It plays a role in many areas, ranging from security devices on credit cards to methods of determining the atomic structure of materials at the nanoscale. Italian scientist Francesco Maria Grimaldi coined the word diffraction and was the first to record accurate observations of the phenomenon in 1660. After that various equivalent formulations were derived; mathematically, diffraction is explained by solving the wave equation for electromagnetic waves, or Schroedinger's equation for matter waves, in some cases with relativistic corrections.
== History ==
The effects of diffraction of light were first carefully observed and characterized by Francesco Maria Grimaldi, who also coined the term diffraction, from the Latin diffringere, 'to break into pieces', referring to light breaking up into different directions. The results of Grimaldi's observations were published posthumously in 1665. Isaac Newton studied these effects and attributed them to inflexion of light rays. James Gregory (1638–1675) observed the diffraction patterns caused by a bird feather, which was effectively the first diffraction grating to be discovered. Thomas Young developed the first wave treatment of diffraction in 1800. In his model Young proposed that the fringes observed behind an illuminated sharp edge arose from interference between the direct transmitted plane wave and a cylindrical wave that appears to emitted from the edge. Augustin-Jean Fresnel revisited the problem and devised an alternative wave theory based on Huygens' principle. In this model, point sources of light are distributed up to the diffraction edge but not in the barrier. These point sources are driven by the incoming plane wave and they interfere beyond the barrier. Fresnel developed a mathematical treatment from his approach and Young's model was initially considered incorrect. Later work showed that Young's more physical approach is equivalent to Fresnel's mathematical one. In 1818, supporters of the corpuscular theory of light proposed that the Paris Academy prize question address diffraction, expecting to see the wave theory defeated. When Fresnel's presentation on his new theory based on wave propagation looked like it might take the prize, Siméon Denis Poisson challenged the Fresnel theory by showing that it predicted light in the shadow behind a circular obstruction. Dominique-François-Jean Arago proceeded to demonstrate experimentally that such light is visible, confirming Fresnel's diffraction model. In 1859 Hermann von Helmholtz and later in 1882 Gustav Kirchhoff developed integral equations for diffraction based on the concepts proposed by Fresnel as well as approximations needed to apply them. In general, all these approaches require formulating the problem in terms of virtual sources. Cases like those with an absorbing barrier require methods developed in the 1940s based on transverse amplitude diffusion.