Past exam of the physics 2 paper of the University of Oxford / 2022 / 5

In charge-free, current-free vacuum, Maxwell's equations areTaking the curl of the third equation and using the fourth, the curl of the curl identity, and the zero divergence of the electric field givesThe magnetic flux density obeys the same wave equation. For the plane wave factor $e^{i(\omega t-kx)}$, the spatial Laplacian contributes $-k^2$ and the second time derivative contributes $-{\omega }^2$. Thusfor propagation in the positive $x$ direction. The zero-divergence conditions require $E_{0x}=B_{0x}=0$, while the curl equations require ${\mathbf{B}}_0=(k/\omega )\hat{\mathbf{x}}\times {\mathbf{E}}_0$. These conditions show that the proposed plane waves satisfy all four Maxwell's equations, rather than only their derived wave equations. Physical electric fields and magnetic fields are the real parts of the complex expressions.

Using $\nabla \times \mathbf{E}=-{\partial }_t\mathbf{B}$ directly with the specified plane wave givesThe electric field, magnetic flux density and propagation direction form a right-handed triad, and for a nonzero wave amplitudeThe electric field and magnetic field have the same wave phase in vacuum.