The reflectances for diffusely incident light on the plane boundary between two non-conducting media are connected by the relation: <i>n</i><sup>2</sup>(1-<i>R</i>)=<i>n</i>′<sup>2</sup>(1-<i>R</i>) where <i>R</i> and <i>R</i>′ are the reflectances for the light, coming from media, having refractive indices <i>n</i> and <i>n</i>′, respectively. This relation may be considered as a particular case of one general photometric invariant.
ANDREW GERSHUN, "Fresnel Reflection of Diffusely Incident Light," J. Opt. Soc. Am. 35, 162-162 (1945)