A scuba diver is floating motionless at a depth below the calm surface of seawater. The seabed lies directly below her. (Assume the speed of sound in seawater is 1480m/s.)
(a) Calculate the minimum depth the diver must have below the water surface for her underwater shout to return from the seabed as a distinct echo.
(b) If the diver is actually at a depth of 20m, determine the time interval between her shout and the echo reflected from the seabed.
(c) Explain briefly why a distinct echo is not heard when the diver is only 5m below the surface, even though the seabed still reflects sound.