Term: reflectance, \(\rho\)
https://doi.org/10.1351/goldbook.R05235

Definition:
Fraction of incident radiation reflected by a surface or discontinuity, ρ λ = P λ refl P λ 0, where Pλ0 and Pλrefl are, respectively, the incident and reflected spectral radiant power.

Note: The reflectance for a beam of light normally incident on a surface separating two materials of refractive indices n 1 and n 2 is given by \[\rho(\lambda) = \frac{\left ( n_{1}\,-\,n_{2}\right )^{2}}{\left ( n_{1}\,+\,n_{2} \right )^{2}}\] Reflectance increases as the @A00346@ of incidence decreases from 90 degrees.

Related Terms:
1) angle (http://doi.org/10.1351/goldbook.A00346).
2) spectral radiant power (http://doi.org/10.1351/goldbook.S05828).

Source: PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 413 (https://doi.org/10.1351/pac200779030293)

Citation: 'reflectance, \(\rho\)' 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.R05235

License: The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms.

Disclaimer: The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using.