$${\frac{E_{r0}^\parallel}{E_{i0}^\parallel} = - \frac{\sin 2 \gamma - \sin 2 \alpha}{\sin 2 \gamma + \sin 2 \alpha} = - \frac{\cos (\alpha + \gamma)}{\sin (\alpha + \gamma)} \frac{\sin (\alpha - \gamma)}{\cos (\alpha - \gamma)}}$$