The ratio of the electric field $(E)$ to the magnetic field $(B)$ in an electromagnetic wave gives which physical quantity?

Answer the following questions:
$(a)$ Long distance radio broadcasts use short-wave bands. Why?
$(b)$ It is necessary to use satellites for long distance $TV$ transmission. Why?
$(c)$ Optical and radio telescopes are built on the ground but $X$-ray astronomy is possible only from satellites orbiting the earth. Why?
$(d)$ The small ozone layer on top of the stratosphere is crucial for human survival. Why?
$(e)$ If the earth did not have an atmosphere,would its average surface temperature be higher or lower than what it is now?
$(f)$ Some scientists have predicted that a global nuclear war on the earth would be followed by a severe 'nuclear winter' with a devastating effect on life on earth. What might be the basis of this prediction?

The velocity factor of a transmission line is $x$. If the dielectric constant of the medium is $2.6$,the value of $x$ is:

$A$ $27\, mW$ laser beam has a cross-sectional area of $10\, mm^2$. The magnitude of the maximum electric field in this electromagnetic wave is given by: $........\, kV/m$. [Given permittivity of free space $\epsilon_0 = 9 \times 10^{-12}\, SI\, units$, speed of light $c = 3 \times 10^8\, m/s$]

In an electromagnetic wave,the maximum value of the electric field is $18 \, V/m$. What is the maximum value of the magnetic field?

The ratio of the amplitude of the magnetic field to the amplitude of the electric field for an electromagnetic wave propagating in vacuum is equal to: