The maximum electron density of the ionosphere layer in the morning is $10^{10} \ m^{-3}$. At noon,the maximum electron density increases to $2 \times 10^{10} \ m^{-3}$. What is the ratio of the critical frequency at noon to the critical frequency in the morning?

Three observers $A, B$ and $C$ measure the speed of light coming from a source. Observer $A$ moves towards the source,observer $C$ moves away from the source with the same speed,and observer $B$ remains stationary. The surrounding space is a vacuum everywhere. Which of the following is true regarding the measured speeds ${v_A}, {v_B}$ and ${v_C}$?

When an $EM$ wave propagates through a vacuum,then:

$A$ magnetic field vector in an electromagnetic wave is represented by $\vec{B} = B_0 \sin(2\pi vt - \frac{2\pi x}{\lambda}) \hat{j}$. Its associated electric field vector is . . . . . . .

For which of the following media will the wave propagation occur without experiencing attenuation?

$A$ plane electromagnetic wave of frequency $25\; GHz$ is propagating in vacuum along the $z$-direction. At a particular point in space and time,the magnetic field is given by $\overrightarrow{B} = 5 \times 10^{-8} \hat{j}\; T$. The corresponding electric field $\overrightarrow{E}$ is (speed of light $c = 3 \times 10^{8}\; m/s$):