Answer :
in the diagram, the ship send sound(?) waves to the water, to determine if there is anything there. If there is something like a sunken ship shown in the diagram, the waves return in a shorter time hence you can understand if theres something or now. This is the principle of radars and sonars.
Explanation :
The full form of SONAR is Sound Navigation and Ranging. It consists of a transmitter and a detector. This effect is used to find the depth of the sea and to know the location of submarines, valleys etc.
The ultrasonic waves move through the water. It strikes the sea bed and reflected back. The detectors detect it.
Let the speed of sound in seawater is v and it takes t time in the transmitting and receiving ultrasonic waves. The total distance in transmitting and receiving signal is 2d.
So, [tex]2d=v\times t[/tex]
Hence, the given figure works on the principle of SONAR.