If the amplitude of an audio signal is half the amplitude of the carrier wave,what is the modulation index?

In amplitude modulation:

The maximum amplitude of an $AM$ wave is $90 \ V$ and the minimum amplitude is $10 \ V$. What is the amplitude of the modulating wave in the given $AM$ wave (in $V$)?

In a modulation process where the pulse duration varies with the modulating signal,it is called ......

The electron density of $E, F_1, F_2$ layers of the ionosphere is $2 \times 10^{11} \, m^{-3}$,$5 \times 10^{11} \, m^{-3}$,and $8 \times 10^{11} \, m^{-3}$ respectively. What is the ratio of critical frequencies for the reflection of radio waves?

To double the covering range of a $TV$ transmission tower,its height should be multiplied by