- Problem solved by Stokes in 1862.
- Can calculate leaf reflectance and transmittance of $N$ layers as a function of a single layer ($R(1)$ and $T(1)$):
$$
\frac{R(N)}{b^{N}-b^{-N}}=\frac{T(N)}{a-a^{-1}}=\frac{1}{ab^{N}-a^{-1}b^{-N}},
$$
where
$$
\begin{align}
a &=\frac{1+R^{2}(1)-T^{2}(1) + \Delta}{2R(1)}\\
b &=\frac{1-R^{2}(1)+T^{2}(1) + \Delta}{2T(1)}\\
\Delta &= \sqrt { (T^{2}(1)-R^{2}(1)-1)-4R^{2}(1)}.
\end{align}
$$