$${2\omega \bigg[ \bigg(t-z' \frac{n(\omega)}{c}\bigg)-(z-z') \frac{n(2\omega )}{c}\bigg]=2\omega \bigg[t-z \frac{n(2\omega)}{c}+z' \frac{n(2\omega )-n(\omega)}{c} \bigg]}$$