The maximum distance up to which $TV$ transmission from a $TV$ tower of height $h$ can be received is proportional to

Which type of medium is used for optical communication?

Choose the correct statement.

$A$ radar has a power of $1 \ kW$ and is operating at a frequency of $10 \ GHz$. It is located on a mountain top of height $500 \ m$. The maximum distance up to which it can detect an object located on the surface of the earth (Radius of earth $= 6.4 \times 10^6 \ m$) is ....... $km$.

$A$ $TV$ tower has a height of $160 \,m$. Its coverage range is nearly (Earth's radius,$R_e = 6400 \,km$):

$A$ carrier signal $60 \sin(2\pi \times 10^6 t)$ is used to modulate an audio signal $15 \sin(300 \pi t)$. The depth of modulation is .......... $\%$.