To radiate $EM$ signal of wavelength $\lambda$ with high efficiency,the antennas should have a minimum size equal to

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?

$A$ point source of electromagnetic radiation has an average power output of $800 \ W$. Find the maximum value of the magnetic field at a distance of $3.5 \ m$ from the source.

If a source is transmitting electromagnetic waves of frequency $8.2 \times 10^6 \ Hz$,then the wavelength of the electromagnetic waves transmitted from the source will be.....$m$.

Out of the following options,which one can be used to produce a propagating electromagnetic wave?

Which of the following is not transported by electromagnetic waves?