  We shall give conditions on the illuminations $\varphi_{i}$ such that the
solutions to Maxwell's equations \[ \left\{ \begin{array}{l} {\rm curl}
E^{i}=i\omega\mu H^{i}\qquad\text{in }\Omega,\\ {\rm curl}
H^{i}=-i(\omega\varepsilon+i\sigma)E^{i}\qquad\text{in }\Omega,\\
E^{i}\times\nu=\varphi_{i}\times\nu\qquad\text{on }\partial\Omega,
\end{array}\right. \] satisfy certain non-zero qualitative properties inside
the domain $\Omega$, provided that a finite number of frequencies $\omega$ are
chosen in a fixed range. The illuminations are explicitly constructed. This
theory finds applications in several hybrid imaging problems, where unknown
parameters have to be imaged from internal measurements. Some of these examples
are discussed. This paper naturally extends a previous work of the author
[Inverse Problems 29 (2013) 115007], where the Helmholtz equation was studied.
