Which characteristic of a carrier light wave is modulated for the modulation of a light wave?

For an amplitude modulated wave,the maximum and minimum amplitudes are $12 \,V$ and $3 \,V$ respectively. Then the modulation index is

Television signals on earth cannot be received at distances greater than $100\, km$ from the transmission station. The reason behind this is that

$A$ $TV$ transmission antenna at a particular station has a height of $100 \, m$. By how much should the height of the antenna be increased to double its coverage range (in $m$)?

The bandwidth of a coaxial cable is approximately ........ .

If the maximum and minimum amplitudes of a modulated wave are $25 \ V$ and $5 \ V$ respectively,then the modulation index is