UPSC Prelims 2011 · GS Paper I
Question 64: Science and Technology
What is the difference between a CFL and an LED lamp ?
- 1.To produce light, a CFL uses mercury vapour and phosphor while an LED lamp uses semi-conductor material.
- 2.The average life span of a CFL is much longer than that of an LED lamp.
- 3.A CFL is less energy-efficient as compared to an LED lamp.
Which of the statements given above is/are correct ?
AnswerThe answer is (c).
A CFL produces light when an electric discharge excites mercury vapour inside the tube and a phosphor coating converts the ultraviolet to visible light. An LED uses a semiconductor that emits light when current passes through it. LEDs use less power for the same light and last far longer, tens of thousands of hours against several thousand for a CFL, and contain no mercury. So statements 1 and 3 are correct and statement 2 is wrong.
- 1. A CFL uses mercury vapour and phosphor; an LED uses a semiconductor.
- 2. An LED lasts much longer than a CFL.
- 3. A CFL uses more power than an LED for the same light.
How you could have got this
- If you were sure statement 2 is not correct, (b) and (d) go: each of them includes 2.
What you needed to know: An LED lasts much longer than a CFL.
- Then, if you were sure statement 3 is correct, (a) goes: it leaves 3 out.
What you needed to know: A CFL uses more power than an LED for the same light.
Only (c) is left. You didn’t need statement 1.
This follows an unofficial answer: UPSC has not published its key for this year, and if that answer is wrong, so is this path.
Which options go is worked out from UPSC’s key and the options, not by a model.
Why each option is right or wrong
- (a) Right on statement 1 but misses statement 3, as CFLs are less efficient than LEDs.
- (b) Statement 2 is wrong: LEDs last longer than CFLs.
- (c) Right: the working principle differs, and a CFL is less energy-efficient.
- (d) Includes statement 2, which reverses the life spans.
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2011, General Studies Paper I, Series A. The paper on upsc.gov.in.