## Wikipedia - Reflectance 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, $$\rho(\lambda) = \frac{P_{\lambda} ^{\text{refl}}}{P_{\lambda} ^{\text{0}}}$$, where $$P_{\lambda}^{\text{0}}$$ and $$P_{\lambda}^{\text{refl}}$$ are, respectively, the incident and reflected @S05828@.
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.
Source:
PAC, 2007, 79, 293. (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 413 [Terms] [Paper]