:<math> R_{\perp} = \frac { \eta_2(\mathbf{ \hat{k} \cdot \hat{n} }) - \eta_1(\mathbf{ \hat{k}'' ^{\prime\prime} \cdot \hat{n} }) } { \eta_2(\mathbf{ \hat{k} \cdot \hat{n} }) + \eta_1(\mathbf{ \hat{k}'' ^{\prime\prime} \cdot \hat{n} }) } = \frac{\eta_2 / \cos\theta'' ^{\prime\prime} - \eta_1 / \cos\theta} {\eta_2 / \cos\theta'' ^{\prime\prime} + \eta_1 / \cos\theta} = \frac{Z_{2\perp} - Z_{1\perp}} {Z_{2\perp} + Z_{1\perp}} </math>
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