1. The apparatus used in a ship to find out the depth of the sea underneath is called _______.   sonar   bell jar   tuning fork   Hint Half-n-half Clue   2. Sound can be reflected as well as refracted.   True   False   3. An echo from the sea-bed is received 1.50 s after an ultrasonic vibration is sent from a sonar on a ship. If the speed of sound in water is 1500 m/s, then the depth of the sea is _______.   1000 m   1125 m   2250 m   4500 m   Half-n-half Clue   4. The lower limit of frequency that can be heard by the average human is about   2 Hz   20 Hz   200 Hz   2000 Hz   Half-n-half Clue   5. Ultrasonic vibrations are used in medicine for imaging internal organs of the human body.   True   False   6. Dolphins use infrasonic vibrations to locate objects and their prey.   True   False   7. Sounds are produced by musical wind-instruments when columns of air are made to vibrate.   True   False