Term: Rayleigh ratio
https://doi.org/10.1351/goldbook.R05159

Definition:
The quantity used to characterize the scattered intensity at the scattering angle θ, defined as R(θ) = iθr2/IfV, where I is the intensity of the incident radiation, iθ is the total intensity of scattered radiation observed at an angle θ and a distance r from the point of scattering and V is the scattering volume. The factor f takes account of polarization phenomena. It depends on the type of radiation employed.   For light scattering, dependent on the polarization of the incident beam, f = 1 for vertically polarized light, f = 1 - cos^2 θ for horizontally polarized light and f = (1 + cos^2 θ)/2 for unpolarized light.   For small-angle neutron scattering, f = 1.   For small-angle X-ray scattering, f ≈ 1, if θ < ca. 5°.  

Notes:
0) The dimension of R(θ) is (length-1).
1) In small-@A00346@ @N04116@ @S05487@ the term cross-section is often used instead of R(θ); the two quantities are identical.
2) An alternative recommended symbol is R(θ).

Related Terms:
1) angle (http://doi.org/10.1351/goldbook.A00346).
2) neutron (http://doi.org/10.1351/goldbook.N04116).
3) scattering (http://doi.org/10.1351/goldbook.S05487).
4) scattering angle (http://doi.org/10.1351/goldbook.S05488).
5) polarization (http://doi.org/10.1351/goldbook.P04712).
6) light scattering (http://doi.org/10.1351/goldbook.L03525).

Source: Purple Book, 1st ed., p. 65 (http://old.iupac.org/publications/books/author/metanomski.html)

Citation: 'Rayleigh ratio' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.R05159

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