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GS Paper IUPSC Prelims 2021
Health & Medicine
Question No. 158Interactive Test Mode
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+2-0.66

Q158. 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 a longer lifespan 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.

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 a longer lifespan 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.

Select the correct option below (or cross out options to eliminate)
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Official Answer Key & Detailed Solution

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Official UPSC Answer KeyUPSC Prelims 2021 • Science and Technology

Correct Answer: Option (C) — 1 and 3 only

Concept & Option Breakdown

Standard Reference Verified

Statements 1 and 3 are correct, while Statement 2 is incorrect:

  • Statement 1 is correct: Sodium lamps emit light omnidirectionally (360 degrees), requiring reflectors, whereas LEDs are directional emitters (typically ~180 degrees).
  • Statement 2 is incorrect: LED lamps possess a significantly longer operating lifespan (typically 50,000+ hours) compared to sodium lamps (typically 15,000–24,000 hours).
  • Statement 3 is correct: High- and low-pressure sodium lamps emit a narrow/monochromatic yellowish-orange spectrum with low Color Rendering Index (CRI), whereas LEDs offer a broad visible spectrum with superior color rendering.
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Question Specifications & Syllabus Mapping

Examination & YearUPSC Civil Services Preliminary Examination 2021 (Q158)
Paper & Marking SchemeGS Paper I • 2 Marks (-0.66 Negative for incorrect response)
Syllabus Subject & TopicScience and Technology → Health & Medicine
Official Answer KeyOption (C) — 1 and 3 only

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Frequently Asked Questions — UPSC Prelims 2021 Q158

What is the official answer for UPSC Prelims 2021 GS Paper I Question 158 (Science and Technology)?

The official UPSC answer key for this 2021 Science and Technology (Health & Medicine) question is Option (C): "1 and 3 only". Correct attempts carry 2 Marks with -0.66 Negative penalty for wrong answers.

Which UPSC Prelims syllabus topic does this 2021 question belong to?

This question belongs to GS Paper I under the subject "Science and Technology" and micro-topic "Health & Medicine". You can explore all repeated questions from Health & Medicine in Acme AI's 3D Prelims PYQ Realm or practice 50 full-length and sectional tests in the APT Prelims Test Series.

How can I solve this Health & Medicine UPSC Prelims question using option elimination?

To solve Health & Medicine questions in UPSC Prelims, identify extreme modifiers, verify core NCERT conceptual definitions, and eliminate mutually contradictory statement pairs before locking your final option.

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