UPSC Prelims 2021 Q72: Science & Technology – Energy & Environment
- (a) 3 only
- (b) 2 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer: (c) 1 and 3 only
Explanation
Sodium Lamps vs LED Lamps for Street Lighting
| Statement | Claim | Verdict | Detail |
|---|---|---|---|
| 1 | Sodium lamps produce light in 360 degrees; LEDs do not | TRUE | Sodium vapour lamps emit light omnidirectionally (360 degrees) so much of it is wasted (reflected off ground back to sky). LEDs are directional (Lambertian) - light emitted in a hemispherical or narrow beam pattern, focused where needed. |
| 2 | Sodium lamps have longer life than LEDs | FALSE | LEDs last 50,000-100,000 hours; sodium lamps last 15,000-24,000 hours. LEDs have LONGER life. |
| 3 | Sodium light is monochromatic; LEDs offer colour advantages | TRUE | Low-pressure sodium lamps emit near-monochromatic yellow light (589 nm) - poor colour rendering (CRI ~0). LEDs offer variable spectrum with high CRI (80+), enabling true colour rendering for safety, wayfinding, security. |
Correct: 1 and 3 only. Answer: (c).
Answer: (c) 1 and 3 only.
Why each option is right or wrong
- (a) Incorrect. Statement 1 (360-degree vs directional) is also true.
- (b) Incorrect. Statement 2 is reversed - LEDs last LONGER, not shorter.
- (c) CORRECT. Statements 1 (sodium omnidirectional vs LED directional) and 3 (sodium monochromatic yellow vs LED variable colour spectrum) are true. Statement 2 is wrong - LEDs actually have LONGER life than sodium lamps.
- (d) Incorrect. Statement 2 is factually wrong (LEDs outlast sodium lamps).
Key point
Sodium lamps = 360-degree omnidirectional + monochromatic yellow (589 nm) + shorter life (15-24k hrs). LEDs = directional + variable spectrum + longer life (50-100k hrs). India's UJALA/SLNP replacing sodium/mercury with LEDs.
Remember
Sodium lamps produce light in 360 degrees (omnidirectional) while LED lamps are directional (statement 1 correct). Sodium light is nearly monochromatic yellow (589 nm) while LEDs offer significant colour advantages (statement 3 correct). LEDs actually have LONGER life than sodium lamps (statement 2 wrong). Answer: (c) 1 and 3 only.
Common mistakes
- Assuming sodium lamps are longer-lasting because they are older technology. LEDs actually outperform them on lifespan.
- Not knowing India's Street Lighting National Programme (SLNP) is replacing conventional (sodium/mercury) with LEDs across cities.
- Missing energy efficiency: LEDs are 30-50% more efficient (lumens/watt) than sodium lamps.
Exam tip
LED advantages over sodium lamps: (i) longer life (50-100k hrs vs 15-24k hrs), (ii) directional (no wasted light), (iii) full-spectrum colour (CRI 80+), (iv) higher efficacy (lumens/watt), (v) instant on/off, (vi) mercury-free. India's SLNP replaced 1.34 crore street lights with LEDs.
Sources
- Bureau of Energy Efficiency (BEE) - LED Street Lighting Standards
- EESL - Street Lighting National Programme (SLNP) Reports