Diffraction

Not to be confused with refraction, the change in direction of a wave passing from one medium to another.

A diffraction pattern of a red laser beam projected onto a plate after passing through a small circular aperture in another plate

Diffraction is the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a secondary source of the propagating wave. Italian scientist Francesco Maria Grimaldi coined the word diffraction and was the first to record accurate observations of the phenomenon in 1660.[1][2]

Infinitely many points (three shown) along length project phase contributions from the wavefront, producing a continuously varying intensity on the registering plate

In classical physics, the diffraction phenomenon is described by the Huygens–Fresnel principle that treats each point in a propagating wavefront as a collection of individual spherical wavelets.[3] The characteristic bending pattern is most pronounced when a wave from a coherent source (such as a laser) encounters a slit/aperture that is comparable in size to its wavelength, as shown in the inserted image. This is due to the addition, or interference, of 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 (e.g., a diffraction grating), a complex pattern of varying intensity can result.

These effects also 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,[4] including gravitational waves,[5] water waves, and other electromagnetic waves such as X-rays and radio waves. Furthermore, quantum mechanics also demonstrates that matter possesses wave-like properties and, therefore, undergoes diffraction (which is measurable at subatomic to molecular levels).[6]

The amount of diffraction depends on the size of the gap. Diffraction is greatest when the size of the gap is similar to the wavelength of the wave. In this case, when the waves pass through the gap they become semi-circular.

  1. ^ Francesco Maria Grimaldi, Physico mathesis de lumine, coloribus, et iride, aliisque annexis libri duo (Bologna ("Bonomia"), Italy: Vittorio Bonati, 1665), page 2 Archived 2016-12-01 at the Wayback Machine:

    Original : Nobis alius quartus modus illuxit, quem nunc proponimus, vocamusque; diffractionem, quia advertimus lumen aliquando diffringi, hoc est partes eius multiplici dissectione separatas per idem tamen medium in diversa ulterius procedere, eo modo, quem mox declarabimus.

    Translation : It has illuminated for us another, fourth way, which we now make known and call "diffraction" [i.e., shattering], because we sometimes observe light break up; that is, that parts of the compound [i.e., the beam of light], separated by division, advance farther through the medium but in different [directions], as we will soon show.
  2. ^ Cajori, Florian "A History of Physics in its Elementary Branches, including the evolution of physical laboratories." Archived 2016-12-01 at the Wayback Machine MacMillan Company, New York 1899
  3. ^ Wireless Communications: Principles and Practice, Prentice Hall communications engineering and emerging technologies series, T. S. Rappaport, Prentice Hall, 2002 pg 126
  4. ^ Suryanarayana, C.; Norton, M. Grant (29 June 2013). X-Ray Diffraction: A Practical Approach. Springer Science & Business Media. p. 14. ISBN 978-1-4899-0148-4. Retrieved 7 January 2023.
  5. ^ Kokkotas, Kostas D. (2003). "Gravitational Wave Physics". Encyclopedia of Physical Science and Technology: 67–85. doi:10.1016/B0-12-227410-5/00300-8. ISBN 9780122274107.
  6. ^ Juffmann, Thomas; Milic, Adriana; Müllneritsch, Michael; Asenbaum, Peter; Tsukernik, Alexander; Tüxen, Jens; Mayor, Marcel; Cheshnovsky, Ori; Arndt, Markus (25 March 2012). "Real-time single-molecule imaging of quantum interference". Nature Nanotechnology. 7 (5): 297–300. arXiv:1402.1867. Bibcode:2012NatNa...7..297J. doi:10.1038/nnano.2012.34. ISSN 1748-3395. PMID 22447163. S2CID 5918772.