MPSC 2022 Prelims Paper-I · Question 39 of 100

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.

• (1)Prenatal examination — 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.
• (2)the study of heart valve action — 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.
• (3)Geological study — 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.

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.

The physics of sound is a standing item in the science component of MPSC papers and this question shows the Commission's preferred method with it: take a technique that everybody has heard of and offer three other applications of the same underlying physics. The question then tests whether the candidate can distinguish a principle from an instrument. That distinction is worth generalising, because the same construction is used elsewhere — a question about radar can offer applications of radio waves, a question about a laser can offer applications of light — and in every case the discriminating step is to read the name of the technique and ask what it was built to do. Here the acronym does the work by itself, since navigation and ranging describes finding a position and a distance and nothing in the option set except the submarines matches it. Two smaller points belong to this particular item. The English stem prints the expansion as Sound Navigational Ranging rather than the standard Sound Navigation and Ranging, and the Marathi block translates ranging by a word meaning series or grade, so the two columns of the paper gloss the acronym differently; neither affects the answer, and a candidate who notices the oddity should not read it as a signal. And the stem is a fill-in-the-blank in both columns, a form this paper uses several times, in which the sentence to be completed carries the question and there is no question mark to alert the reader.

• SONAR stands for Sound Navigation and Ranging, and it is a technique for determining the position and distance of objects under water by transmitting ultrasonic pulses and detecting their echoes; this paper prints the expansion as Sound Navigational Ranging.
• Because the pulse travels to the object and back, the distance is the speed of sound in the water multiplied by the recorded time and then halved; the speed of sound in seawater is of the order of fifteen hundred metres per second and varies with temperature, salinity and depth.
• Sonar is used to measure the depth of the sea, map the shape of the sea bed, and locate submerged objects including submarines, sunken ships, icebergs and shoals of fish.
• Ultrasound is sound above about twenty thousand hertz, inaudible to human beings, and useful for probing because its short wavelength resolves small objects and it can be directed in a narrow beam.
• Other techniques share sonar's principle but are not sonar: medical ultrasonography images the interior of the body, echocardiography examines the heart and its valves, ultrasonic testing detects flaws in metals, and seismic surveying maps subsurface geology; bats and dolphins use the same principle naturally, which is called echolocation.

• Reasoning that any application of ultrasound is sonar, when ultrasound is the tool and sonar is one particular instrument built with it for navigation and ranging under water
• Confusing sonar with sonography because the words resemble each other, when the second is a medical imaging technique working over centimetres inside the body
• Forgetting to halve the product of speed and time when computing a distance from an echo, since the pulse covers the distance twice
• Reading a fill-in-the-blank stem as an incomplete sentence and looking for a question elsewhere on the page, a form this paper uses on several questions

Sound is examined in MPSC science sections both as recall — define ultrasound, state where sound travels fastest, name the technique used for a stated purpose — and as short numerical work based on the echo relation or on the wave equation linking speed, frequency and wavelength. The recall variant used here is built by surrounding the right application with other applications of the same physics, so the discriminating knowledge is what a named technique is for rather than how it works. A candidate who prepares this chapter should be able to say in one sentence each what sonar, ultrasonography, echocardiography and ultrasonic flaw detection do, and should hold the speed of sound in air and in water as approximate figures, since a numerical question will normally supply the speed but a recall question may test it. Expect at least one question on sound or on light in every science section.

No directly related past PYQ was found.

• practice — not a real PYQ

  A sonar pulse sent from a ship returns from the sea bed after 4 seconds. If the speed of sound in seawater is taken as 1500 metres per second, what is the depth of the sea at that point ?

  • (a)6000 m
  • (b)3000 m
  • (c)375 m
  • (d)750 m

  Answer(b) 3000 m. The pulse travels down to the sea bed and back, so in 4 seconds it covers 1500 multiplied by 4, which is 6000 metres, and the depth is half of that, namely 3000 metres. The option of 6000 metres is what a candidate obtains by forgetting to halve, which is the standard error in echo problems, and it is placed first for that reason. Always ask whether the quoted time is for a one-way journey or for a round trip before using it.

• practice — not a real PYQ

  Which of the following correctly distinguishes sonar from medical ultrasonography, given that both use ultrasound ?

  • (a)Sonar uses audible sound while ultrasonography uses ultrasound
  • (b)Sonar determines the position and distance of objects under water, while ultrasonography produces images of structures inside the body
  • (c)Ultrasonography relies on reflection while sonar relies on refraction
  • (d)Sonar works only in air while ultrasonography works only in water

  Answer(b) Sonar determines the position and distance of objects under water, while ultrasonography produces images of structures inside the body. Both rest on the same relation between the speed of sound, the time taken by an echo and the distance travelled, and both use ultrasound; what separates them is the medium, the scale and the output — a range measured over hundreds or thousands of metres of water in one case, an image assembled over a few centimetres of tissue in the other.