v 5.4

$${\begin{align*}E_1^* E_2 + E_1 E_2^* &= E_{10}E_ {20} [\exp(i(\varphi_1 - \varphi_2)) + \exp(-i(\varphi_1 - \varphi_2))] \\&= 2E_{10}E_{20}\cos (\varphi_1 - \varphi_2)\end{align*}}$$