Question 77 from UPSC CSE 2021 - S&T

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.

Accepted Answer

Correct Option: C. 1 and 3 only

Strategy: Evaluate each statement based on the physical and operational characteristics of sodium lamps versus LEDs. Retain statements that accurately describe the omnidirectional light emission and monochromatic spectrum of sodium lamps. Discard the statement claiming sodium lamps have a longer lifespan, as LEDs are more durable. The correct statements are 1 and 3.

Key Info: **Sodium Lamps**
- Emit light **omnidirectionally** (360 degrees).
- Produce a **monochromatic** yellow-orange spectrum, resulting in poor color rendering.
- Offer lower lifespans (LPS: ~18,000 hours; HPS: ~24,000 hours).
- Have been used for street lighting due to low initial cost and good luminous efficacy.
- Drawbacks include inefficient light direction and frequent maintenance.

**LEDs (Light Emitting Diodes)**
- Emit light in a **controlled, directional** pattern (typically 120-180 degrees), minimizing light loss and pollution.
- Feature high energy efficiency and low heat output.
- Offer significantly longer lifespans, often exceeding 50,000–100,000 hours.
- Provide **broad-spectrum** emission with high Color Rendering Index (CRI) for vivid, accurate color perception.
- Include benefits like instant-on capability, dimming, and no hazardous materials.
- The transition to LED lighting is driven by maintenance savings, improved illuminance, and reduced ecological impact.

Answer

Option A: 3 only

Answer

Option B: 2 only

Answer

Option D: 1, 2, and 3