An electromagnetic wave having frequency $4 \times 10^{14} \,Hz$ is passing through a small volume. The energy contained in this volume oscillates with frequency

Electromagnetic waves are incident normally on a perfectly reflecting surface having surface area $A$. If $I$ is the intensity of the incident electromagnetic radiation and $c$ is the speed of light in vacuum,the force exerted by the electromagnetic wave on the reflecting surface is

$A$ plane electromagnetic wave is incident on a plane surface of area $A$ normally,and is perfectly reflected. If energy $E$ strikes the surface in time $t$,then the average pressure exerted on the surface is ($c=$ speed of light).

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?

An electromagnetic wave travels in a medium with a speed of $2 \times 10^8 \ m/s$. The relative permeability of the medium is $1$. Then the relative permittivity is:

$A$ plane electromagnetic wave propagates along the $+x$ direction in free space. The components of the electric field,$\overrightarrow{E}$ and magnetic field,$\overrightarrow{B}$ vectors associated with the wave in a Cartesian frame are: