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$${\frac{E_{r0}^\parallel}{E_{i0}^\parallel} = - \frac{N_{21}^2 \cos \alpha - i \sqrt{ \sin^2 \alpha - N_{21}^2}}{N_{21}^2 \cos \alpha + i \sqrt{\sin^2 \alpha - N_{21}^2}}\\ \frac{E_{r0}^\perp}{E_{i0}^\perp} = \frac{ \cos \alpha -i \sqrt{\sin^2 \alpha - N_{21}^2}}{\cos \alpha + i \sqrt{\sin^2 \alpha - N_{21}^2}}}$$