What is $SONAR$?

How does a stretched string,on being set into vibration,produce an audible sound?

Define echo. Establish a mathematical relation between speed of sound, distance of reflecting body from source of sound, and time for echo.

$(a)$ $A$ particular transmitter of Aakashvani broadcasts at $420.5\, m$ wavelength. Given the speed of radio waves $3 \times 10^{8}\, m s^{-1}$. Calculate the frequency at which the radio station broadcasts its programme.
$(b)$ What is the direction of oscillations of the medium particles through which a
$(i)$ transverse wave is propagating?
$(ii)$ longitudinal wave is propagating?

Define the terms:
$(i)$ vibration
$(ii)$ time period
$(iii)$ frequency and
$(iv)$ amplitude. [one mark each]

State whether the following statement is true or false:
The wavelength of $5\, MHz$ ultrasound in a medium where the velocity is $1540\, m s^{-1}$ is $0.3\, mm$.