In a plane electromagnetic wave,the electric field oscillates with a frequency $2 \times 10^{10} \,s^{-1}$ and amplitude $40 \,Vm^{-1}$. The energy density due to the electric field is (given $\varepsilon_0 = 8.85 \times 10^{-12} \,Fm^{-1}$):

$A$ point light source emits electromagnetic ($E$.$M$.) waves in free space. $A$ detector,placed at a distance of $L \text{ m}$,measures the intensity as $I_o$. The detector is now shifted to another location on the same spherical surface,ensuring the angle between the original location and the new location is $45^{\circ}$. The measured intensity at the new location will be . . . . . . .

The electron density of $E, F_1,$ and $F_2$ layers of the ionosphere is $4 \times 10^{11} \, m^{-3}, 9 \times 10^{11} \, m^{-3},$ and $16 \times 10^{11} \, m^{-3}$ respectively. What is the ratio of the critical frequencies for the reflection of radio waves?

$A$ plane electromagnetic wave of frequency $28 \, MHz$ travels in free space along the positive $x$-direction. At a particular point in space and time, the electric field is $9.3 \, V/m$ along the positive $y$-direction. The magnetic field (in $T$) at that point is:

$A$ sky wave with a frequency $55 \, MHz$ is incident on the $D$-region of the Earth's atmosphere at $45^\circ$. The angle of refraction is ......... $^\circ$ (electron density for the $D$-region is $400 \, \text{electrons}/cm^3$).

The speed of electromagnetic radiation in vacuum is