Editing Redshift (section)

==Effects from physical optics or radiative transfer==
The interactions and phenomena summarized in the subjects of [[radiative transfer]] and [[physical optics]] can result in shifts in the wavelength and frequency of electromagnetic radiation. In such cases, the shifts correspond to a physical energy transfer to matter or other photons rather than being by a transformation between reference frames. Such shifts can be from such physical phenomena as [[Wolf effect|coherence effects]] or the [[scattering]] of [[electromagnetic radiation]] whether from [[electric charge|charged]] [[elementary particle]]s, from [[particulates]], or from fluctuations of the [[index of refraction]] in a [[dielectric]] medium as occurs in the radio phenomenon of [[Whistler (radio)|radio whistlers]].<ref name=basicastronomy/> While such phenomena are sometimes referred to as "redshifts" and "blueshifts", in astrophysics light-matter interactions that result in energy shifts in the radiation field are generally referred to as "reddening" rather than "redshifting" which, as a term, is normally reserved for the [[#Redshift formulae|effects discussed above]].<ref name=basicastronomy/>

In many circumstances scattering causes radiation to redden because [[entropy]] results in the predominance of many low-[[energy]] photons over few high-energy ones (while [[conservation of energy|conserving total energy]]).<ref name=basicastronomy/> Except possibly under carefully controlled conditions, scattering does not produce the same relative change in wavelength across the whole spectrum; that is, any calculated {{math|''z''}} is generally a [[function (mathematics)|function]] of wavelength. Furthermore, scattering from [[randomness|random]] [[matter|media]] generally occurs at many [[angle]]s, and {{math|''z''}} is a function of the scattering angle. If multiple scattering occurs, or the scattering particles have relative motion, then there is generally distortion of [[spectral line]]s as well.<ref name=basicastronomy/>

In [[interstellar medium|interstellar astronomy]], [[Visible spectrum|visible spectra]] can appear redder due to scattering processes in a phenomenon referred to as [[interstellar reddening]]<ref name=basicastronomy/>—similarly [[Rayleigh scattering]] causes the [[Atmosphere of Earth|atmospheric]] reddening of the Sun seen in the sunrise or sunset and causes the rest of the sky to have a blue color. This phenomenon is distinct from red''shift''ing because the [[spectroscopic]] lines are not shifted to other wavelengths in reddened objects and there is an additional [[extinction (astronomy)|dimming]] and distortion associated with the phenomenon due to photons being scattered in and out of the [[Line-of-sight propagation|line of sight]].<ref>{{Cite web |last=Impey |first=Chris |editor-last=Gay |editor-first=Pamela |title=Dust Extinction and Reddening |url=https://www.teachastronomy.com/textbook/The-Milky-Way/Dust-Extinction-and-Reddening/ |access-date=2025-03-06 |website=Teach Astronomy - Dust Extinction and Reddening |publisher=Teach Astronomy |language=en}}</ref>