In an electromagnetic wave,the amplitude of the electric field is $1 \ V/m$ and the frequency of the wave is $5 \times 10^{14} \ Hz$. The wave propagates in the positive $Z$-direction. The average energy density of the electric field in this wave is ........ $J \ m^{-3}$.

When $\mu_r$ and $\epsilon_r$ are the relative permeability and relative permittivity (dielectric constant) respectively,the refractive index is given by:

The ratio of contributions made by the electric field and magnetic field components to the intensity of an electromagnetic wave is:

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 magnetic field in a travelling electromagnetic wave has a peak value of $20 \ nT$. The peak value of electric field strength is ...... $Vm^{-1}$.

The amplitude of the magnetic field in an electromagnetic wave propagating along the $y$-axis is $6.0 \times 10^{-7} \, T$. The maximum value of the electric field in the electromagnetic wave is: