2010 General Studies, all questions

UPSC Prelims 2010 · General Studies

Question 108: Science and Technology

Indiscriminate disposal of used fluorescent electric lamps causes mercury pollution in the environment. Why is mercury used in the manufacture of these lamps ?

aA mercury coating on the inside of the lamp makes the light bright white
bWhen the lamp is switched on, the mercury in the lamp causes the emission of ultra-violet radiations
cWhen the lamp is switched on, it is the mercury which converts the ultra-violet energy into visible light
dNone of the statement given above is correct about the use of mercury in the manufacture of fluorescent lamps

AnswerThe answer is (b).

In a fluorescent lamp, an electric discharge through low-pressure mercury vapour makes the mercury atoms emit ultraviolet radiation. The phosphor coating on the inner wall of the tube converts that ultraviolet into visible light. Mercury, a toxic metal, is thus needed to produce the ultraviolet, which is why discarded lamps pollute. Option (c) wrongly gives mercury the job of the phosphor. Remember: mercury makes ultraviolet; the phosphor makes visible light.

Why each option is right or wrong
  • (a) The inner coating is phosphor, not mercury.
  • (b) Right: mercury emits ultraviolet radiation when the lamp is switched on.
  • (c) The phosphor, not the mercury, converts ultraviolet into visible light.
  • (d) Statement (b) is correct, so 'none' is wrong.

Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2010, General Studies, Series A. The paper on upsc.gov.in.