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 ✓ UPSC's answer
- (d) 1, 2 and 3
Why the answer is (c)
• Statement 1 is correct: a sodium-vapour arc tube emits light in all directions (360°), needing reflectors, whereas LEDs are directional (about 180°), so more light reaches the road.
• Statement 3 is correct: low-pressure sodium light is nearly monochromatic yellow (589 nm) with poor colour rendering, while LEDs give white light with good colour rendering, improving visibility and CCTV imaging.
• Statement 2 is wrong: LEDs last 50,000–100,000 hours, far longer than sodium lamps (about 15,000–24,000 hours).
• India's Street Lighting National Programme (EESL) replaced over a crore sodium lamps with LEDs on these grounds.
• Hence 1 and 3 only, option (c).
Why the other options are wrong
- (a) 3 only
- Statement 1 is also correct: sodium lamps emit in 360°.
- (b) 2 only
- Statement 2 is wrong: LEDs outlast sodium lamps.
- (d) 1, 2 and 3
- Statement 2 is wrong.
Asked in the GS Paper I of the UPSC Civil Services Preliminary Examination 2021, held on 10 October 2021. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.