The reflectances for diffusely incident light on the plane boundary between two non-conducting media are connected by the relation: n2(1-R)=n′2(1-R) where R and R′ are the reflectances for the light, coming from media, having refractive indices n and n′, 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)