Question

Define modulation index. Why is it kept low ? What is the role of a band pass filter ?

Answer 1

Modulation index is defined as the ratio of the amplitude of the modulating wave to that of the carrier wave.

So, $\mu =E_m/E_c$.

The modulation index is kept low because when $E_m$ is greater than $E_c$ the carrier wave is over-modulated, $\mu$ > 1, which causes distortion during the reception.
The main function of a bandpass filter is to limit the bandwidth of the output signal to the band allocated for the transmission.