UPSC Prelims 2021 · GS Paper I
Question 72: Science and Technology
With reference to street-lighting, how do sodium lamps differ from LED lamps?
- 1.Sodium lamps produce light in 360 degrees but it is not so in the case of LED lamps.
- 2.As street-lights, sodium lamps have longer life span than LED lamps.
- 3.The spectrum of visible light from sodium lamps is almost monochromatic while LED lamps offer significant colour advantages in street-lighting.
Select the correct answer using the code given below.
AnswerUPSC’s answer is (c).
Sodium lamps radiate light in all directions (360 degrees), while LEDs emit directionally, so light can be aimed at the road. LEDs last far longer than sodium lamps, so statement 2 is wrong. Low-pressure sodium lamps give an almost monochromatic yellow light, whereas LEDs offer good colour rendering and a choice of colour temperatures, which is a significant advantage in street-lighting.
- 1. Sodium lamps emit in all directions; LEDs are directional.
- 2. LEDs have a longer life span than sodium lamps.
- 3. Sodium light is nearly monochromatic yellow; LEDs give better colour options.
How you could have got this
- If you were sure statement 1 is correct, (a) and (b) go: each of them leaves 1 out.
What you needed to know: Sodium lamps emit in all directions; LEDs are directional.
- Then, if you were sure statement 2 is not correct, (d) goes: it includes 2.
What you needed to know: LEDs have a longer life span than sodium lamps.
Only (c) is left. You didn’t need statement 3.
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) Wrong: statement 1 is also correct.
- (b) Wrong: LEDs have the longer life, so statement 2 is false.
- (c) Right: statements 1 and 3 are correct.
- (d) Wrong: statement 2 is false.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2021, General Studies Paper I, Series A. The paper on upsc.gov.in.