$${E_t = E_{t0} \exp\bigg[-\frac{2\pi}{\lambda_2} z \sqrt{\frac{\sin^2 \lambda }{N_{21}^2 }-1} \bigg] \exp\bigg[i \omega \bigg( t - \frac{x}{N_{21} v_2/\sin\alpha} )\bigg) \bigg]}$$