Authentic Classroom Explanation by IAS Monk

💡 Q8. Sodium Lamps vs LED Lamps

📌 Question
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:

(a) 3 only
(b) 2 only
(c) 1 and 3 only
(d) 1, 2 and 3

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✅ Correct Answer: (c) 1 and 3 only

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🧑‍🏫 Authentic Classroom Explanation by IAS Monk

From first principles to final option — the UPSC way.

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🔬 Concept First

Street-light efficiency depends on:

• Direction of light emission
• Energy efficiency
• Colour rendering ability
• Operational lifespan

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🔍 Statement-wise Analysis

Statement 1:
Sodium lamps produce light in 360°, LEDs do not
✔️ Correct

• Sodium vapour lamps are omnidirectional
• Much light is wasted and must be reflected
• LEDs emit light directionally, reducing losses

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Statement 2:
Sodium lamps have longer lifespan than LEDs
❌ Incorrect

• Typical sodium lamp lifespan: ~20,000–25,000 hours
• LED lifespan: 50,000–100,000 hours
• LEDs drastically reduce maintenance costs

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Statement 3:
Sodium lamps are almost monochromatic; LEDs offer colour advantages
✔️ Correct

• Sodium lamps emit yellow-orange light
• Poor colour rendering (objects appear greyish)
• LEDs provide broad spectrum light with high CRI

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🔍 Curiosity Raiser

Did you know?
Under sodium streetlights, a red car and a blue car can look almost identical at night.

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🧠 Enrichment Notes (UPSC Lens)

• Why cities are switching to LEDs:
  • Energy savings
  • Better visibility
  • Lower maintenance
  • Reduced light pollution
• Common UPSC Trap:
  • Assuming older technology lasts longer (❌)
• Exam Value:
  • Application of physics to urban infrastructure & sustainability

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🪶 IAS Monk Closing Whisper

Light is not just about brightness, but about direction and truth.