Physics & chemistry in everyday life

Answer: (b) 2 and 3 only

A black box holds the flight data recorder and the cockpit voice recorder, and it is built from tough materials such as steel and titanium. Its locating aid at sea is an underwater beacon that sends an acoustic signal at 37.5 kHz, not red light.

• ✗ 1. The underwater locator beacon on flight recorders is a 'pinger' that sends an acoustic (sound) signal at 37.5 kHz once it is immersed in water. It is not a red-light beacon.
• ✓ 2. A flight recorder combines a flight data recorder and a cockpit voice recorder. The Air India AI171 investigation describes recorders that store digital flight data and cockpit voice on the same device.
• ✓ 3. DD News (Prasar Bharati), reporting on the Air India AI171 recorders, says the device is constructed from highly durable materials like steel and titanium, so the memory units are made of stainless steel or titanium.

Remember · Black box: records flight data and cockpit voice; built from durable steel or titanium; its underwater locator beacon sends a 37.5 kHz acoustic signal, not light.

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Answer: (c) Only three

In a lithium-ion cell the cathode is a metal-oxide material built from lithium with nickel, cobalt (and manganese), as in the common NMC type. Graphite is the anode material, not part of the cathode. So three of the four elements make up cathodes.

• ✓ I Cobalt is one of the metals in the active cathode material.
• ✗ II Graphite forms the anode of the cell, not the cathode.
• ✓ III Lithium is a cathode mineral (the IEA lists lithium, nickel, cobalt and manganese for the cathode).
• ✓ IV Nickel is a main cathode metal; nickel-rich cathodes are a key trend in EV batteries.

Remember · EV battery cathode: lithium with nickel, cobalt (and manganese). Anode: graphite.

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Answer: (c) All the three

All three contain plastic. Cigarette filters are made mainly of cellulose acetate fibres, which are microplastics; many eyeglass lenses are plastic (for example CR-39 and polycarbonate); and tiny plastic particles are shed by car tyres as they wear.

• ✓ I The WHO Framework Convention on Tobacco Control Secretariat says cigarette filters are mainly composed of microplastics known as cellulose acetate fibres.
• ✓ II Many spectacle lenses are made of plastic: a peer-reviewed study of spectacle lens materials covers CR-39, polycarbonate and Trivex, and calls CR-39 lenses the most basic form of plastic lenses.
• ✓ III IUCN counts particles worn off car tyres among the tiny plastic particles that reach the oceans.

Remember · Cigarette filters (cellulose acetate), eyeglass lenses (CR-39, polycarbonate) and car tyres (which shed microplastics) all contain plastic.

Question and answer: UPSC's official GS Paper I (2025, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 1 Oct 2026 (how we verify). Permalink · Report an error

Answer: (b) Only two

Coal gasification turns coal into syngas, which is then converted into fuels and chemicals. India's Ministry of Coal lists ethanol among these products and describes the Talcher plant converting syngas to ammonia and then urea. Nitroglycerine is made by nitrating glycerine with nitric and sulphuric acids and is not one of the coal-gasification products, so only two of the three qualify.

• ✓ I The Ministry of Coal includes ethanol among the liquid fuels made from syngas and notes that a policy change is under way to include ethanol from the coal gasification route.
• ✗ II Nitroglycerine (glyceryl trinitrate) comes from nitrating glycerine with mixed nitric and sulphuric acids; it is not a syngas product.
• ✓ III At Talcher, coal blended with pet-coke is gasified to syngas, converted into ammonia and then into urea.

Remember · Coal gasification gives syngas, then methanol, ethanol, ammonia and urea (Talcher fertiliser plant). Nitroglycerine comes from glycerine and nitrating acids, not from syngas.

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Answer: (a) Both Statement II and Statement III are correct and both of them explain Statement I

Both Statement II and Statement III are correct, and both explain Statement I. A large surface area lets activated carbon hold heavy metals and other pollutants on its surface, and because it can be made cheaply from carbon-rich waste, it becomes an attractive tool for cleaning effluents.

• ✓ II Activated carbon is highly porous, with a very large surface area and strong adsorption capacity. Pollutants stick to that surface (adsorption), which is exactly how it strips heavy metals from wastewater. This explains why it removes pollutants (Statement I).
• ✓ III Farm and other carbon-rich wastes are the usual raw material for activated carbon, and the process is simple and cheap. This low cost explains why it is 'attractive' as a treatment tool (Statement I).
• ✗ (b) This option says only one of II and III explains Statement I. But the ability to adsorb (II) explains the 'good' part and the cheap, waste-based supply (III) explains the 'attractive' part, so both explain it.
• ✗ (d) Both statements are supported by published research on biomass-derived activated carbon, so 'neither is correct' fails.

Remember · Activated carbon = porous, huge surface area, strong adsorbent, cheaply made from carbon-rich waste; hence widely used to clean effluents.

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Answer: (a) Both Statement I and Statement II are correct and Statement II explains Statement I

Both statements are correct. Rare earth compounds such as europium and yttrium oxides act as phosphors, the light-emitting coatings that give colour in television and computer screens, so their luminescent property is exactly why they are used in displays.

• ✓ Statement-I IREL (India) Limited, a Department of Atomic Energy undertaking, lists television screens, computers and monitor displays among rare-earth uses; europium and yttrium oxides produce the red colour in television screens.
• ✓ Statement-II Rare earths have luminescent properties, and several (europium, terbium) are used as phosphors. That is why they light up screens, so this statement explains Statement-I.

Remember · Europium and yttrium oxides are rare-earth phosphors: their luminescence gives red (and, with europium, blue) colour in TV and phone screens.

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Answer: (a) silver iodide and potassium iodide

Artificial rain is made by cloud seeding: seed particles are added to a cloud so that rain can form, and Delhi has trialled it as an emergency measure against pollution episodes. India's IITM names silver iodide as the standard seeding agent for cold clouds, and UPSC's key pairs it with potassium iodide.

• ✓ (a) Both agents are iodides. IITM's CAIPEEX pages name silver iodide (AgI) as the ice-nucleating agent used in glaciogenic seeding; UPSC's key names potassium iodide as the second agent, which the IITM pages do not mention.
• ✗ (b) Silver nitrate replaces silver iodide here; the key's answer has both agents as iodides.
• ✗ (c) Potassium nitrate replaces potassium iodide here; the key's answer has both agents as iodides.
• ✗ (d) Silver nitrate and potassium chloride replace both iodides.

Remember · Cloud seeding uses iodide salts, chiefly silver iodide (with potassium iodide in the key), to trigger rain.

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Answer: (d) 1, 2 and 3

PFAS are man-made fluorinated chemicals used for non-stick, stain-proof and grease-proof products. Their carbon–fluorine bonds make them extremely persistent ('forever chemicals'), so they spread through water, food and packaging and build up in the bodies of people and animals. All three statements hold.

• ✓ 1. PFAS turn up in drinking water, in foods such as fish and dairy, and in grease-resistant food packaging like fast-food wrappers and pizza boxes.
• ✓ 2. They are long-lasting chemicals whose components break down very slowly, so they persist in the environment.
• ✓ 3. Because they persist, many PFAS are found in the blood of people and animals worldwide, and some accumulate in the body over time.

Remember · PFAS = 'forever chemicals': very slow to break down, found in water, food and packaging, and accumulate in human and animal bodies.

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Answer: (d) Statement-I is incorrect, but Statement-II is correct

A star's mass decides how fast it burns. Big, massive stars need very high rates of nuclear fusion in their cores to hold themselves up against their own gravity, so they use up their hydrogen in a few million years, while small dwarf stars burn slowly and can shine for billions or even trillions of years. So the fusion-rate statement is true, and the longer-life statement is the reverse of the truth.

• ✗ Statement-I Massive stars are short-lived: some last only a few million years, whereas low-mass red dwarfs can last trillions of years; the Sun will spend about 10 billion years on the main sequence.
• ✓ Statement-II More massive stars must fuse hydrogen at a much higher rate to generate the outward pressure that balances their stronger gravity, which also makes them hotter and brighter.

Remember · Bigger star → faster fusion → shorter life. Massive stars: millions of years, end as supernovae; red dwarfs: trillions of years. Sun: about 10 billion years on the main sequence.

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Answer: (a) 1 only

Hydrogels are water-swollen networks of cross-linked polymers, soft and compatible with body tissue. Their best-established use is medical: a drug trapped in the gel is released over time at a controlled rate, and the gels are also used in wound dressings, contact lenses and tissue engineering. UPSC's key treats statements 2 and 3 as incorrect.

• ✓ 1. Hydrogels can hold small drugs, proteins or even cells and release them over time and at a chosen site, and several such delivery systems are in clinical use.
• ✗ 2. UPSC's official key treats this statement as incorrect; we could not confirm the detail from an official source, so we do not explain it here.
• ✗ 3. UPSC's official key treats this statement as incorrect; we could not confirm the detail from an official source, so we do not explain it here.

Remember · Hydrogel = cross-linked, water-absorbing polymer network; soft, biocompatible. Uses: controlled drug release, wound dressings, soft contact lenses, tissue engineering.

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Answer: (d) Water vapour

A hydrogen fuel cell combines hydrogen from the tank with oxygen from the air to produce electricity for the motor. The only chemical product of that reaction is water (H2O), so the tailpipe of a fuel-cell car releases just water vapour and some warm air, with no smog-forming pollutants.

• ✓ (d) At the cathode, the hydrogen ions, the electrons that have passed through the circuit and oxygen join to form water, which leaves as vapour.
• ✗ (c) Oxygen is consumed by the fuel cell, not given out; it is taken in from the air as a reactant.
• ✗ (a) Hydrogen peroxide (H2O2) is not a product of a fuel cell's hydrogen–oxygen reaction; the cell is designed to yield plain water.

Remember · Hydrogen fuel cell: 2H2 + O2 → 2H2O + electricity + heat. FCEV tailpipe emits only water vapour and warm air.

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Answer: (a) 1 only

Only statement 1 is correct. Carbon fibre is a light, strong material used in vehicle bodies and in aircraft parts, and it can be recovered from used composites and reused, so it is not a one-time material.

• ✓ 1. Carbon-fibre composites go into vehicle chassis, body panels and doors, and into aircraft fairings, fuselages and floor panels. Their strength for low weight is the reason.
• ✗ 2. Carbon fibre can be recycled. Recycled fibre is already used in new products and needs far less energy than making virgin fibre, which is highly energy intensive.

Remember · Carbon fibre is strong and light, used in cars and aircraft, and is recyclable: recovered fibre is reused in new composites.

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Answer: (c) All three

All three actions need an accelerometer. An accelerometer is a sensor that measures changes in motion, and a crash, a free fall and a tilt each show up as a change in acceleration that the device can read.

• ✓ 1. Airbag systems decide whether to deploy from how abruptly the car's speed changes, which an accelerometer measures.
• ✓ 2. Drives in laptops use an accelerometer to sense free fall; the drive then parks its read-write head or powers down before impact to avoid damage.
• ✓ 3. A phone's accelerometer senses the direction of gravity, so it knows how the phone is tilted and rotates the display between portrait and landscape.

Remember · An accelerometer senses changes in motion. It triggers airbags, protects laptop drives in free fall and rotates phone screens.

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Answer: (c) Infrared waves are largely absorbed by water vapour that is concentrated in the lower atmosphere.

Statement (c) is correct: water vapour, ozone and other gases in the lower atmosphere (the troposphere) absorb much of the near-infrared radiation passing through. The other options misstate insolation values or the nature of infrared light.

• ✓ (c) NCERT says water vapour, ozone and other gases within the troposphere absorb much of the near-infrared radiation. Water vapour is concentrated in the lower atmosphere.
• ✗ (a) NCERT gives insolation at the surface as about 320 W/m2 in the tropics and about 70 W/m2 at the poles, a ratio of under five, and it says the equator receives less than the tropics. Ten times is far too high.
• ✗ (d) Infrared waves lie just beyond the visible spectrum; the human eye cannot see them. They are not part of the visible spectrum.

Remember · In the troposphere, water vapour, ozone and other gases absorb much of the near-infrared radiation. Insolation is about 320 W/m2 in the tropics and 70 W/m2 at the poles.

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Answer: (a) 1 and 3

Statements 1 and 3 are correct: PET is the polyester of clothing, blended with cotton and wool, and PET bottles are recycled, mostly into polyester fibre. Statement 2 is wrong because PET lets in oxygen and is unsuited to long storage of some alcoholic drinks. Statement 4 is wrong because burning PET turns its carbon into carbon dioxide, a greenhouse gas.

• ✓ 1. PET fibres are used alone or blended with natural fibres such as cotton, and wool-polyester and cotton-polyester blends are common textile mixes.
• ✗ 2. PET has only moderate oxygen permeability, so it suits short storage but harms the aroma of an alcoholic drink such as wine when kept for long. It is not fit for any and every alcoholic beverage.
• ✓ 3. PET bottles are mechanically recycled; in India most recycled PET, about 95% by one estimate, is spun into polyester fibre.
• ✗ 4. Burning plastic releases CO2 in proportion to its carbon. EPA's model takes PET as 63% carbon, with about 98% of that carbon turning into CO2 on combustion.

Remember · PET = polyester: blended with cotton and wool, and recycled (mostly into fibre). Burning it releases CO2, and it is a poor container for long-stored alcoholic drinks.

Question and answer: UPSC's official GS Paper I (2022, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 1 Oct 2026 (how we verify). Permalink · Report an error

Answer: (c) 1 and 3 only

The cooking temperature is the boiling point of water at the pressure held inside the cooker, and that pressure is fixed by the weight of the lid's regulator and the area of the vent hole it covers. A hotter flame only makes steam faster; it does not raise the cooking temperature.

• ✓ 1. Steam escapes through the vent hole once its push (pressure × hole area) exceeds the regulator's weight. So the hole area helps decide how much pressure builds up, and water boils at a higher temperature at higher pressure.
• ✗ 2. A bigger flame makes steam sooner, but once the regulator lifts, the extra steam simply escapes. The pressure, and hence the boiling temperature, stays the same.
• ✓ 3. The weight placed over the vent is what the steam must lift. A heavier weight lets more pressure build up, so the water boils at a higher temperature (well above 100 °C).

Remember · Water boils at a higher temperature under higher pressure. In a pressure cooker the vent weight and vent-hole area set the pressure; the flame only sets how fast it is reached.

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Answer: (a) it is dipolar in nature

Water dissolves so many substances because its molecule is dipolar (polar): the oxygen end carries a slight negative charge and the hydrogen end a slight positive charge. These charged ends attract ions and other polar molecules and pull them apart, so they dissolve.

• ✓ (a) The two charged ends of a water molecule attract the particles of many solids, such as the ions in common salt, and separate them into the liquid.
• ✗ (b) Conducting heat well has nothing to do with dissolving. Water is in fact a poor conductor of heat compared with metals.
• ✗ (c) High specific heat means water needs a lot of heat to change its temperature. It explains slow heating and cooling, not solvent power.
• ✗ (d) Being an oxide of hydrogen only describes water's composition (H2O). Many other compounds are oxides and dissolve little or nothing.

Remember · Water is called the universal solvent because its molecule is polar (dipolar), with a positive and a negative end that pull other substances apart.

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Answer: (c) 1 and 3 only

Sodium lamps throw light out in all directions and give a narrow spectrum weighted towards yellow and red, whereas LEDs emit from one side of the chip in a directional beam and can produce broad-spectrum white light. LEDs also last longer than sodium lamps, so statement 2 is the wrong one.

• ✓ 1. Lamp-type sources, such as sodium lamps, give light in nearly all directions, so reflectors and lenses are needed to send it to the road. An LED emits from one side of its chip and is directional.
• ✗ 2. LED products typically last far longer than conventional lamps, with good-quality ones rated at 30,000 to 50,000 hours or more. Sodium lamps do not outlast them.
• ✓ 3. Sodium lamps are narrow-spectrum sources that put most of their output into longer wavelengths such as yellow and red, so the visible light is almost monochromatic. LEDs can give a broad white spectrum, which is the colour advantage the statement claims.

Remember · Against sodium lamps, LED street lights are directional, longer-lasting, more efficient and broad-spectrum; sodium lamps radiate in all directions with a narrow yellow-red-weighted spectrum.

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Answer: (b) Polycarbonate

BPA is the building-block chemical of polycarbonate, the hard, clear plastic used in reusable bottles and food containers. It is also used in epoxy resins, such as the lining of food cans. The official agencies name only these two plastics as its main uses, so the other options fail.

• ✓ (b) Polycarbonate is made by joining BPA units, so BPA is a structural part of it. Small amounts can leach into food or drink, which is why BPA is a health concern (it can act on the hormone system).
• ✗ (a) Low-density polyethylene (carry bags and films) is not among the plastics that US and EU agencies name as made with BPA; they name polycarbonate and epoxy resins.
• ✗ (c) PET, the plastic of soft-drink and water bottles, is not among the plastics that US and EU agencies name as made with BPA; polycarbonate is the BPA-based bottle plastic.
• ✗ (d) Polyvinyl chloride is not among the plastics that US and EU agencies name as made with BPA; they name polycarbonate and epoxy resins.

Remember · BPA goes into polycarbonate plastics and epoxy resins (can linings). It is a suspected hormone disruptor.

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Answer: (d) Speed of light is always same.

A light-year is the distance light covers in one year, and it works as a fixed yardstick because the speed of light in vacuum is constant. Space distances are so large that a unit with a fixed, known value is far more convenient than kilometres.

• ✓ (d) Light travels at about 3 × 10^8 m/s in vacuum, always the same value. So the distance it covers in one year is a fixed length, roughly 9 trillion km.
• ✗ (a) Distances between stars and galaxies do change, because they move; the light-year is chosen for another reason.
• ✗ (c) Light does not always go straight: it bends when it passes from one medium to another, so straight-line travel is not the basis of the unit.
• ✗ (b) The strength of gravity of a star has no role in defining a unit of length.

Remember · A light-year is a distance (about 9.46 trillion km), not a time. It is usable because the speed of light in vacuum is constant.

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Answer: (c) 1, 3 and 4 only

VLC sends data by rapidly varying visible light, roughly 380 to 780 nm in wavelength, from LEDs. It is a high-speed, indoor, short-range technology, so statement 2 (long-range) is the wrong one; the other three statements are correct.

• ✓ 1. VLC works in the visible part of the spectrum, taken as about 380-780 nm in the ITU standard; UPSC's 375 nm lower limit is a marginal variation of the same band.
• ✗ 2. VLC is designed for high-speed indoor, in-premises use, not for long-range links. It is short-range optical wireless communication.
• ✓ 3. Light offers a far wider bandwidth than radio waves, and a peer-reviewed review lists a higher transmission rate and high bandwidth among VLC's advantages over radio-frequency links, which is why it can carry large amounts of data faster than Bluetooth.
• ✓ 4. UPSC's key counts this as correct: VLC carries data on light, not on radio waves, so it is not subject to radio-frequency electromagnetic interference (the review lists reduced interference among VLC's advantages over radio).

Remember · VLC (Li-Fi) = data on visible light (about 380-780 nm): fast, short-range, indoor, free of radio interference.

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Answer: (c) 1, 3 and 4 only

Carbon nanotubes are tiny hollow tubes of rolled-up carbon sheets that are used in medicine to carry drugs and in biochemical sensors, and living systems can break them down with enzymes. Making artificial blood capillaries from them is not an established use, so statement 2 is the one that fails.

• ✓ 1. Carbon nanotubes are used as carriers, for example to deliver drugs and other payloads in the body, and are studied for bioimaging as well.
• ✗ 2. The established medical uses are bioimaging, drug delivery and biosensing. Building artificial blood capillaries for an injured body part is not one of them.
• ✓ 3. Biosensing is one of the main biomedical uses of functionalised carbon nanotubes.
• ✓ 4. Enzymes such as peroxidases can degrade carbon nanotubes, so they can be cleared from the body after delivering their payload; research is now focused on this route.

Remember · Carbon nanotubes: drug carriers and biosensors, and enzyme-degradable; not used as artificial blood capillaries.

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Answer: (b) 'Gravitational waves' were detected.

The merger of two black holes was significant because it produced gravitational waves, ripples in space-time that scientists detected directly for the first time on 14 September 2015 using the LIGO detectors. The waves, predicted by Einstein a hundred years earlier, travelled for 1.3 billion years before reaching Earth.

• ✓ (b) The Nobel Prize in Physics 2017 press release says the first observed gravitational waves came from a collision between two black holes. An Indian team was part of the group that announced the discovery in 2016.
• ✗ (c) NASA states that black holes do not provide shortcuts between different points in space, so nothing about travel through wormholes was confirmed.
• ✗ (a) The Higgs boson is a particle searched for in particle-physics experiments; the black-hole merger observation was about gravitational waves, not that particle.
• ✗ (d) The observation is remembered for detecting gravitational waves, not for explaining the 'singularity' inside a black hole.

Remember · First direct detection of gravitational waves (LIGO, 14 September 2015): two black holes merging about 1.3 billion light-years away; Nobel Prize in Physics 2017.

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Answer: (d) 1, 2 and 3

All three follow from Einstein's General Theory of Relativity (1915-16). It treats gravity as the curving of space-time by matter, so matter warps space-time, and light, which follows that curved space, is bent by gravity. Applied to the whole universe, the same equations allow an expanding universe.

• ✓ 1. A massive body such as the Sun curves space-time, so light passing it no longer travels in a straight line. This bending was tested during the solar eclipse of 29 May 1919.
• ✓ 2. Einstein's theory suggested that the universe expands. Einstein himself doubted this and added a 'cosmological constant' to avoid it; Friedmann's solutions of the equations and Hubble's observations supported expansion.
• ✓ 3. This is the core idea: mass and energy curve the space-time around them, and objects, including light, move along that curvature.

Remember · General Relativity: matter curves space-time, and that curvature is gravity. It bends light and allows an expanding universe.

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Answer: (a) Observation and understanding of the Universe

All four terms belong to physics and cosmology, the study of how the Universe is built and how it began. An event horizon and a singularity are parts of a black hole, and the Standard Model describes matter and its forces.

• ✓ (a) The event horizon is a black hole's boundary, inside which nothing can escape, and the singularity is the point at its centre where matter is crushed to infinite density. The Standard Model explains how the building blocks of matter interact. String theory is a physicists' theory of fundamental forces and matter. All four help us understand the Universe.
• ✗ (b) Eclipses are explained by the positions of the Sun, Earth and Moon and need none of these terms.
• ✗ (c) Launching satellites concerns rockets and orbits, not black holes or particle physics.
• ✗ (d) The origin of living organisms belongs to biology and evolution. NCERT ties 'singularity' to the Big Bang, the origin of the Universe.

Remember · Event horizon and singularity are black-hole terms; the Standard Model is particle physics; string theory is theoretical physics. All concern the Universe.

📘 Read it in NCERT: Class 11 Fundamentals of Physical Geography, Ch 2 (practise this chapter)

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Answer: (d) 1, 2 and 3

All three pairs are correct. Each names a chemical that has drawn regulatory or health concern in an everyday product: lead in cosmetics such as lipstick, brominated vegetable oil in soft drinks, and added monosodium glutamate (MSG) in Chinese-style fast food.

• ✓ 1. Lead is a toxic heavy metal that can turn up as a contaminant in cosmetics. Indian rules bar lead compounds as colouring in cosmetics and Indian Standards set a 20 ppm ceiling for lead.
• ✓ 2. Brominated vegetable oil (BVO) is a clouding agent that has been used in soft drinks. WHO's food-additive expert committee (JECFA) allocated it no acceptable daily intake, and FSSAI's manual carries a test to detect BVO in soft drinks.
• ✓ 3. MSG is a flavour enhancer commonly added in Chinese-style cooking. It is controversial enough that FSSAI requires a label on foods with added MSG, saying it is not recommended for infants below 12 months and pregnant women.

Remember · Lead in lipstick, brominated vegetable oil in soft drinks, MSG in Chinese fast food: all three are chemicals of health concern, so all pairs match.

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Answer: (c) 1, 2 and 3

All three statements are advantages of OLED displays. Each OLED pixel makes its own light, so the panel needs no rigid backlight unit as an LCD does, and it can be built thin, on flexible plastic, see-through or rolled up.

• ✓ 1. OLED layers can be deposited on flexible substrates, not only on glass, which is why bendable and foldable phone screens use them. An LCD needs a backlight and a rigid liquid-crystal cell.
• ✓ 2. Because the panel can be made on a flexible base, roll-up and wearable displays, including ones stitched into clothing, are possible. Curved, bendable, foldable and rollable OLED screens already exist.
• ✓ 3. OLEDs can be made transparent, so a see-through display is possible. An LCD depends on a backlight behind the screen, which blocks that.

Remember · OLED pixels emit their own light (no backlight), so screens can be thin, flexible, rollable and transparent. LCDs cannot do this easily.

Question and answer: UPSC's official GS Paper I (2017, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error

Answer: (b) To detect gravitational waves

eLISA was planned as a space-based observatory to detect gravitational waves, the ripples in space-time caused by events such as merging black holes. It works as a laser interferometer in space, so its purpose is gravitational-wave detection.

• ✓ (b) The name gives it away: a Laser Interferometer Space Antenna is a laser-based gravitational-wave detector in space, the space counterpart of ground detectors such as LIGO.
• ✗ (a) Neutrinos are detected by large underground or under-ice detectors, not by a laser interferometer in space.
• ✗ (c) A missile-defence test has nothing to do with this science mission.
• ✗ (d) Solar flares are studied by solar observatories and space-weather missions, not by an interferometer built for gravitational waves.
• • Since then The eLISA concept is now the European Space Agency's LISA mission, described as the first space-based observatory for gravitational waves, with launch planned for 2035.

Remember · eLISA/LISA = laser interferometer in space to detect gravitational waves. Ground-based cousin: LIGO, which India is joining.

📘 Read it in NCERT: Class 11 Knowledge Traditions and Practices of India, Part I, Ch 5 (practise this chapter)

Question and answer: UPSC's official GS Paper I (2017, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink · Report an error