## reflectance, $$\rho$$

Also contains definitions of: reflectance factor, reflectivity
https://doi.org/10.1351/goldbook.R05235
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 @[email protected] of incidence decreases from 90 degrees.
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)