(a) When a boy strikes an iron pipe, sound is produced. The produced sound wave travels through air and through the pipe. Since speed of sound in iron is greater than the speed of sound in air, so the sound travels faster through iron pipe than through air. Therefore, another boy at the end of the pipe hears two sounds one which travelled through iron pipe and the other which travelled through air.
(b) (i) Ultrasonic waves are used to deter¬mine the depth of a sea. It is done with the help of SONAR.
(ii) Ultrasonic waves are used for diagnosing diseases in human body.