In the following question, out of four words given below, three are alike in some manner and fourth word is different.

Find the different one.

Question

In the following question, out of four words given below, three are alike in some manner and fourth word is different. Find the different one.

UnlockIAS · Accepted Answer

Correct — C, STETHOSCOPE. All four words end in -SCOPE, so the setter has deliberately removed the easiest classifier: a feature shared by four options out of four can separate none of them. The classifier that does work is function. A microscope forms a magnified image of something too small for the eye, using an objective and an eyepiece whose powers multiply — a 40x objective with a 10x eyepiece gives 400x. A telescope gathers light from something too far away, either by refraction through lenses (Hans Lippershey's patent application of 1608, Galileo's instrument of 1609) or by reflection off a curved mirror (Newton's reflector, 1668). A periscope sees past an obstacle: two plane mirrors, or two right-angle prisms, are fixed parallel at 45 degrees to the tube, each turning the light through 90 degrees, so the line of sight is shifted bodily along the tube without changing its direction — the arrangement that let submarine crews and trench soldiers look up and out while staying down. All three end at the eye and all three work with light. The stethoscope does not. Rene Laennec built the first one at the Necker Hospital in Paris in 1816 by rolling sheets of paper into a cylinder, and what it carries is sound, not light: the chest-piece picks up heart, breath and bowel sounds and the tubing conducts them to the ear. The clinical act is auscultation — listening. Nothing is ever seen through a stethoscope, which is why it is the one that does not belong.

Why the other options are wrong — (a) An optical instrument you look through: lenses refract light to form a magnified image of an object too small for the naked eye, and total magnification is the objective power multiplied by the eyepiece power. It shares the group's defining feature — an image delivered to the eye — so it cannot be the different one. Candidates who choose it are usually splitting the set by distance ('three examine things that are far off or hidden, one examines something near'), a split that collapses because a periscope also looks at objects only a few metres away.  (b) Also optical, and also ending at the eye: lenses in a refractor, of the kind Galileo turned on Jupiter in 1610, or a curved primary mirror in a reflector after Newton's 1668 design, collect light from a distant object and bring it to a focus the eye can use. Being the most familiar word in the set makes it a tempting 'safe' pick for a candidate who is guessing rather than classifying, but familiarity is not a category, and on the light-versus-sound test the telescope sits squarely with the microscope and the periscope.  (d) Optical as well, and mechanically the most interesting of the three: two plane mirrors or right-angle prisms set parallel at 45 degrees reflect the incoming ray twice through 90 degrees, displacing the line of sight along the length of the tube — the principle of the submarine periscope and of the trench periscopes of the First World War. This is the wrong answer the set is built to attract, because it is the least familiar word and therefore feels odd. Unfamiliarity is a feeling, not a classification; the instrument still delivers an image to the eye.

Concept: An odd-one-out item is built on two features, one superficial and one deep. The superficial feature is chosen to cover all four options so that it can decide nothing — here, the suffix -SCOPE. The deep feature is chosen to cover exactly three — here, forming an image with light. Your task is never to notice what the four have in common; it is to find the single attribute under which the set splits three-to-one. The linguistics is part of the trap and is worth knowing. The suffix comes from the Greek skopein, 'to look at, to examine', and most instruments carrying it are indeed visual: microscope, telescope, periscope, endoscope, kaleidoscope, ophthalmoscope. But when Rene Laennec named his 1816 invention he took the broader sense, 'examine', and joined it to stethos, 'chest'; the stethoscope therefore examines the chest without looking at anything at all. That is exactly why a classification by word form fails here and a classification by function succeeds. The physics behind the split is equally clean. Optical instruments work by refraction through lenses or reflection from mirrors and need no material medium at all, while an acoustic instrument conducts pressure waves and cannot work without one — which is the reason a stethoscope has to be pressed against the patient while the other three work across a gap.

State PCS 2025 Q1 — Science & Technology PYQ with Answer

State PCS 2025 Q1 — Science & Technology PYQ with Answer