What is the range of frequency of $EM$ waves that are reflected back by the ionosphere?

Why is the orientation of the portable radio with respect to the broadcasting station important?

The condition under which a microwave oven heats up a food item containing water molecules most efficiently is

The refractive index of the ionosphere is ........

The radiation energy emitted per second by a point source is $100 \,W$. If the efficiency of the source is $4 \%$, then the rms value of the electric field at a distance of $2 \,m$ is [use $\frac{1}{4 \pi \varepsilon_0}=9 \times 10^9$ in $SI$ units].

$A$ plane electromagnetic wave propagating in the direction of the unit vector $\hat{n}$ with a speed $c$ is described by electric and magnetic field vectors $E$ and $B$,respectively. Which of the following relations (in $SI$ units) between $E$ and $B$ can be ruled out on dimensional grounds alone?