State whether the following statement is true or false:
Light waves require no medium for their propagation.

$A$ vibrating tuning fork is placed over the mouth of a burette filled with water. The tap is opened and the water level gradually falls. It is observed that the sound becomes the loudest for a particular length of air column.
$(i)$ What is the name of the phenomenon taking place when this happens?
$(ii)$ Why does the sound become the loudest?
$(iii)$ How does the frequency of the loud sound compare with that of the tuning fork?
$(iv)$ State the unit for measuring loudness.

Bats have no eyes,yet they can ascertain the distance,direction,nature,and size of objects. Explain why.

$(i)$ What does the acronym $SONAR$ stand for?
$(ii)$ What is the audible range of sound for human beings?
$(iii)$ How do the bats search and catch their prey in dark night?

Give two applications of ultrasound.

An echo is heard after $0.8 \, s$,when a person fires a cracker $132.8 \, m$ from a high building. Calculate the speed of sound.