I am solving exercises where I occasionally need to decompose the radiance of an EM wave in its components, but I don't understand how it's done. In this context, reflectance ($R$) and transmissivity ($T$), defined as power ratios derived from Fresnel's laws, should always verify that $R+T=1$ due to the conservation of energy. In the case I'm having trouble with, where the electric field is neither $p-$ nor $s-$polarized, but at an angle $\theta$ with the incidence plane of a wave that interacts with an interphase between two dielectric materials, my professor decomposes R as:
$$R = R_s \sin^2 \alpha + R_p \cos^2 \alpha$$
If $R_s$ and $R_p$ could be seen as components of a vector (which I guess is not possible), there would need to be a $^2$ exponent in $R_s$, $R_p$ and $R$. But this doesn't seem to be the case, and I am confused as to why this happens. Does this have anything to do with the requirement that $R+T = 1$, or is this a mistake on my professor's behalf?