📐 波動方程式
$$\frac{\partial^2 E_x}{\partial z^2} = \mu_0 \epsilon_0 \frac{\partial^2 E_x}{\partial t^2}$$
$$\frac{\partial^2 B_y}{\partial z^2} = \mu_0 \epsilon_0 \frac{\partial^2 B_y}{\partial t^2}$$
解: $E_x = E_0 \cos(kz - \omega t)$, $B_y = B_0 \cos(kz - \omega t)$
関係: $c = \omega/k = 1/\sqrt{\mu_0\epsilon_0}$