{{short description|Frequency shift in the electromagnetic spectrum}} The '''Wolf effect''' (sometimes '''Wolf shift''') is a frequency shift in the electromagnetic spectrum.<ref>Emil Wolf, "[http://www.worldscibooks.com/physics/4331.html Selected Works of Emil Wolf: With Commentary]" (2001) [https://books.google.com/books?id=WK0-TUHxy0EC&dq=wolf++redshift+%22new+mechanism%22&pg=PA638 p.638], {{ISBN|981-02-4204-2}}.</ref> The phenomenon occurs in several closely related phenomena in radiation physics, with analogous effects occurring in the scattering of light.<ref name="james">{{cite journal | last=James | first=Daniel F V | title=The Wolf effect and the redshift of quasars | journal=Pure and Applied Optics: Journal of the European Optical Society Part A | publisher=IOP Publishing | volume=7 | issue=5 | year=1998 | issn=0963-9659 | doi=10.1088/0963-9659/7/5/006 | pages=959–970|arxiv=astro-ph/9807205| bibcode=1998PApOp...7..959J | s2cid=17670250 }}</ref> It was first predicted by Emil Wolf in 1987<ref name="wolf87nature">{{cite journal | last=Wolf | first=Emil | title=Non-cosmological redshifts of spectral lines | journal=Nature | publisher=Springer Science and Business Media LLC | volume=326 | issue=6111 | year=1987 | issn=0028-0836 | doi=10.1038/326363a0 | pages=363–365| bibcode=1987Natur.326..363W | s2cid=4337925 }}</ref><ref>{{cite journal | last=Wolf | first=Emil | title=Redshifts and blueshifts of spectral lines caused by source correlations | journal=Optics Communications | publisher=Elsevier BV | volume=62 | issue=1 | year=1987 | issn=0030-4018 | doi=10.1016/0030-4018(87)90057-5 | pages=12–16| bibcode=1987OptCo..62...12W | doi-access=free }}</ref> and subsequently confirmed in the laboratory in acoustic sources by Mark F. Bocko, David H. Douglass, and Robert S. Knox,<ref>{{cite journal | last1=Bocko | first1=Mark F. | last2=Douglass | first2=David H. | last3=Knox | first3=Robert S. | title=Observation of frequency shifts of spectral lines due to source correlations | journal=Physical Review Letters | publisher=American Physical Society (APS) | volume=58 | issue=25 | date=1987-06-22 | issn=0031-9007 | doi=10.1103/physrevlett.58.2649 | pages=2649–2651| pmid=10034809 | bibcode=1987PhRvL..58.2649B }}</ref> and a year later in optic sources by Dean Faklis and George Morris in 1988.<ref>{{cite journal | last1=Faklis | first1=Dean | last2=Morris | first2=G. Michael | title=Spectral shifts produced by source correlations | journal=Optics Letters | publisher=The Optical Society | volume=13 | issue=1 | date=1988-01-01 | issn=0146-9592 | doi=10.1364/ol.13.000004 | page=4—6| pmid=19741961 | bibcode=1988OptL...13....4F }}</ref>
==Theoretical description== In optics, two non-Lambertian sources that emit beamed energy can interact in a way that causes a shift in the spectral lines. It is analogous to a pair of tuning forks with similar frequencies (pitches), connected together mechanically with a sounding board; there is a strong coupling that results in the resonant frequencies getting "dragged down" in pitch. The Wolf Effect requires that the waves from the sources are partially coherent - the wavefronts being partially in phase. Laser light is coherent while candlelight is incoherent, each photon having random phase. It can produce either redshifts or blueshifts, depending on the observer's point of view, but is redshifted when the observer is head-on.<ref name="wolf87nature" />
For two sources interacting while separated by a vacuum, the Wolf effect cannot produce shifts greater than the linewidth of the source spectral line, since it is a position-dependent change in the distribution of the source spectrum, not a method by which new frequencies may be generated. However, when interacting with a medium, in combination with effects such as Brillouin scattering it may produce distorted shifts greater than the linewidth of the source.
==Notes== <div style="font-size: 95%"> {{Reflist}} </div>
Category:Scattering Category:Spectroscopy
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