v 4.16

$${\frac{E_{r0}^\perp}{E_{i0}^\perp} = \frac{n_1 \ cos \alpha - n_2 \cos \gamma}{n_1 \cos \alpha + n_2 \cos \gamma} = \frac{ \cos \alpha - N_{21} \cos \gamma}{\cos \alpha + N_{21} \cos \gamma}}$$