SONAR (Sound Navigational Ranging) technique is used for ________ .

Question

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Correct — option (4). SONAR is an acronym, and reading it out settles the question: the letters stand for Sound Navigation and Ranging, which is a technique for finding where something is and how far away it is by means of sound. The paper prints the expansion as Sound Navigational Ranging, and the Marathi block renders the word ranging by a word meaning series or grade, but the standard expansion of the acronym is Sound Navigation and Ranging and the sense is unaffected. The technique works in water and only in water, which is the second thing the acronym implies once one remembers that sound travels well through a liquid and very badly through the air over long distances, while light and radio waves are absorbed within a short distance underwater. A sonar installation carries a transmitter that emits a pulse of ultrasonic sound, meaning sound of a frequency above the range of human hearing, and a receiver, called a detector, that picks up the pulse after it has been reflected from an object below. The instrument records the time between transmission and return. Since the pulse travels to the object and back again, it has covered twice the distance in that time, so the distance is the speed of sound in seawater multiplied by the recorded time and then halved; the speed used is of the order of fifteen hundred metres per second, though it varies with the temperature, the salinity and the depth of the water. From that single measurement the technique yields the depth of the sea below a ship, the shape of the sea floor with its ridges and valleys, and the position of anything solid suspended in the water — a shoal of fish, a wreck, the submerged part of an iceberg, or a submarine. Option (4) names precisely that last application, the measuring of the motion and position of submerged objects such as submarines, and it is the only one of the four options that describes a method of locating an object under water. The reason the other three options are attractive is that all of them involve ultrasound in some form, and a candidate who has learnt that sonar uses ultrasound may reason that any use of ultrasound is sonar. That inference is the trap. Ultrasound is the tool; sonar is one particular instrument built with that tool for navigation and ranging under water, and medical imaging and industrial testing are different instruments built with the same tool for different purposes. Option (4) is the answer.

Why the other options are wrong — (a) Prenatal examination is carried out by ultrasonography, commonly called sonography, which is a medical imaging technique: a transducer sends ultrasonic pulses into the body, the echoes returning from the boundaries between tissues of different density are detected, and the machine assembles them into an image on a screen. It shares its physical basis with sonar — both send out ultrasound and read the echo — and that is exactly why it appears here. It is nevertheless a different technique with a different name, a different purpose and a different scale, working over a few centimetres inside the human body rather than over hundreds or thousands of metres of ocean, and producing a picture rather than a range. The similarity of the words sonar and sonography does the rest of the work for the examiner. In India there is a further reason to know what prenatal ultrasound is and is not, since its misuse for determining the sex of a foetus is prohibited by law.  (b) The study of the action of the heart valves is echocardiography, which is again a medical application of ultrasound rather than sonar. An ultrasonic beam is directed at the heart and the echoes from the moving valve leaflets and chamber walls are used to produce an image and, with the Doppler technique, to measure the speed and direction of the blood flowing through the valves. The word echo in the name shows that the underlying principle is the same as the one sonar uses, which is what makes the option plausible, but the instrument, its frequencies, its scale and its purpose are those of clinical diagnosis. This is the second of the two medical options in the set, and having two of them is deliberate: it maximises the chance that a candidate who knows only that sonar involves ultrasound will settle on one of them.  (c) Geological investigation does make use of elastic and ultrasonic waves, so this option cannot be dismissed as physically absurd, and that is what makes it the subtlest of the three. But the techniques involved have their own names and their own instruments: seismic surveying sends elastic waves into the ground and records their return at arrays of geophones to map subsurface structures, and ultrasonic testing is used to detect flaws inside metal blocks and castings. Neither is called sonar, because sonar is defined by what its acronym says — navigation and ranging — and by the medium it operates in, which is water. The question asks what the SONAR technique is used for, and among the four options only one describes the locating of an object beneath the surface of the sea. A method that shares a principle with sonar is not thereby sonar.

Concept: Sound is a mechanical wave, which means it needs a material medium and travels by disturbing the particles of that medium. Its speed depends on the medium and is greatest in solids, smaller in liquids and smallest in gases: roughly three hundred and forty metres per second in air at ordinary temperature, about fifteen hundred metres per second in seawater, and several thousand in steel. Sound above about twenty thousand hertz is called ultrasound, is inaudible to human beings, and has two properties that make it useful for probing: its short wavelength lets it resolve small objects, and it can be sent out in a narrow beam rather than spreading in all directions. Every technique that uses ultrasound to find something rests on the same relation. A pulse is emitted, it reflects from a boundary, the echo returns, and because the pulse has travelled the distance twice, the distance equals the speed in the medium multiplied by the total time and divided by two. What differs between the techniques is the medium, the scale and the purpose. Sonar applies the relation in water, over distances from a few metres to several kilometres, to determine the depth of the sea, the shape of the sea bed and the position of submerged objects including submarines, wrecks, icebergs and shoals of fish; it is the reason a ship can know the depth beneath its keel continuously. Medical ultrasonography applies the same relation inside the human body over a few centimetres to build an image, and echocardiography is its application to the heart. Ultrasonic testing applies it to metal to find internal flaws. Nature arrived at the principle long before engineers did: bats and dolphins navigate and hunt by emitting ultrasonic calls and interpreting the returning echoes, a faculty called echolocation, and sonar is sometimes introduced through that comparison.

State PCS 2022 Q39 — Science & Technology PYQ with Answer

State PCS 2022 Q39 — Science & Technology PYQ with Answer