UPPSC 2022 Prelims GS Paper-I · Question 74 of 150

Correct — C, Activity of radio-active substance – Curie. The SI unit of radioactivity is the BECQUEREL, symbol Bq, defined as one nuclear disintegration per second and named after Henri Becquerel, who discovered radioactivity in 1896. The curie is the older unit it replaced: it was pegged to the activity of about one gram of radium-226 and equals 3.7 x 10^10 becquerel, which is why activities quoted in curies are enormous numbers when converted. The curie survives in medicine and industry out of habit, but it is not an SI unit, and that is exactly the confusion this pair is built on. The other three pairs hold: pressure is measured in pascal and heat, being a form of energy, in joule, both SI derived units with special names; the dioptre used for the power of a lens is simply the reciprocal metre under another name, 1 D = 1 m^-1, so it is dimensionally the SI expression for lens power.

• (a)Power of lense – Diopter — Correctly matched, so it is not the answer. The power of a lens is the reciprocal of its focal length in metres, so its unit is the reciprocal metre, and 'dioptre' is that unit under a familiar name — a lens of focal length 0.5 m has a power of +2 D. A converging lens carries a positive power and a diverging lens a negative one, which is why spectacle prescriptions are written with signs.
• (b)Pressure – Pascal — Correctly matched, so it is not the answer. The pascal is the SI derived unit of pressure, one newton per square metre, or kg m^-1 s^-2 in base units. Everyday pressures are usually quoted in bar, atmosphere or millimetres of mercury — none of which is SI — but the pascal is the SI unit, and it is what the question asks for.
• (d)Heat – Joule — Correctly matched, so it is not the answer. Heat is energy in transit, so it takes the SI unit of energy, the joule, which is one newton-metre or kg m^2 s^-2. The calorie is the older unit still used for food energy — one calorie is about 4.18 joule — but like the curie it is a legacy unit, not the SI one.

The International System of Units builds every measurement out of seven base units and expresses everything else as derived units, some of which are given special names — pascal for pressure, joule for energy, watt for power, becquerel for radioactivity, gray for absorbed dose, sievert for equivalent dose. Alongside these sits a large family of legacy units that remain in daily use but are not part of the SI: the calorie, the bar, the atmosphere, the angstrom, the rad, the rem and the curie. Each has a fixed conversion to its SI counterpart. The three radiation quantities are the ones most often mixed up: activity is how fast a source decays (becquerel), absorbed dose is how much energy a body actually takes in (gray), and equivalent dose weights that energy by the type of radiation to give a measure of biological harm (sievert).

A 'NOT correctly matched' unit question is answered by asking of each pair not 'is this unit used for this quantity' but 'is this the SI unit'. Curie, calorie, bar and angstrom will all pass the first test and fail the second, and examiners plant exactly those. Here three pairs are unambiguous SI usages, and the fourth names a unit that any physics text explicitly describes as the non-SI predecessor of the becquerel — so the answer is fixed even without recalling the conversion factor. The pattern is worth generalising: whenever a unit is named after a person from an earlier era of the science and has an awkward conversion factor attached to it, suspect that it has been superseded.

• The SI unit of radioactivity (activity) is the becquerel, Bq — one disintegration per second
• The curie is the non-SI legacy unit it replaced: 1 Ci = 3.7 x 10^10 Bq, roughly the activity of one gram of radium-226
• Pascal = one newton per square metre; joule = one newton-metre; dioptre = one reciprocal metre
• The other SI radiation units are the gray for absorbed dose and the sievert for equivalent dose, replacing the older rad and rem respectively
• Being in common use does not make a unit SI — calorie, bar, atmosphere, angstrom, rad, rem and curie are all outside the system

• Treating the curie as the SI unit of radioactivity; the SI unit is the becquerel and the curie is its non-SI predecessor
• Assuming that a familiar unit must be an SI unit — calorie, bar, atmosphere and angstrom all fail that test
• Confusing the gray (absorbed dose) with the sievert (equivalent dose), or either with the becquerel (activity)

UPPSC sets this as a four-pair 'NOT correctly matched' quantity-to-unit list and expects one legacy unit to be spotted among three SI ones. UPSC has asked the same content as a full four-way List I (Quantity) with List II (Units) match, mixing SI units such as pascal and joule with non-SI ones such as the mach number and the angstrom, so practise both the pair-elimination and the matching format.

• practice — not a real PYQ

  The SI unit of the activity of a radioactive substance is the

  • (a)Curie
  • (b)Becquerel
  • (c)Roentgen
  • (d)Gray

  Answer(b) Becquerel — one disintegration per second. The curie is the older non-SI unit (1 Ci = 3.7 x 10^10 Bq), while the gray measures absorbed dose, not activity.

• practice — not a real PYQ

  The power of a lens is expressed in dioptre. One dioptre is equivalent to

  • (a)one metre
  • (b)one reciprocal metre
  • (c)one square metre
  • (d)one reciprocal second

  Answer(b) one reciprocal metre — lens power is the reciprocal of the focal length measured in metres, so a lens of focal length 0.5 m has a power of +2 D.