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.
Answer & explanation
Answer: (c) 1 and 3 only
Sodium lamps throw light out in all directions and give a narrow spectrum weighted towards yellow and red, whereas LEDs emit from one side of the chip in a directional beam and can produce broad-spectrum white light. LEDs also last longer than sodium lamps, so statement 2 is the wrong one.
- ✓ 1. Lamp-type sources, such as sodium lamps, give light in nearly all directions, so reflectors and lenses are needed to send it to the road. An LED emits from one side of its chip and is directional.
- ✗ 2. LED products typically last far longer than conventional lamps, with good-quality ones rated at 30,000 to 50,000 hours or more. Sodium lamps do not outlast them.
- ✓ 3. Sodium lamps are narrow-spectrum sources that put most of their output into longer wavelengths such as yellow and red, so the visible light is almost monochromatic. LEDs can give a broad white spectrum, which is the colour advantage the statement claims.
Remember · Against sodium lamps, LED street lights are directional, longer-lasting, more efficient and broad-spectrum; sodium lamps radiate in all directions with a narrow yellow-red-weighted spectrum.
Sources
- US Department of Energy, LED Street Lighting (Solid-State Lighting Program posting, 21 June 2016) ↗ “By contrast, conventional lamp-based technologies produce light in all directions, so more than half of the output is typically redirected toward the desired target by means of reflectors and lenses.”
- US Department of Energy, LED Basics ↗ “LED lighting products typically last far longer than their conventional counterparts.”
- US Department of Energy / PNNL, An Investigation of LED Street Lighting's Impact on Sky Glow (2017) ↗ “Beginning around the 1970s, more narrow- spectrum products like high-pressure sodium (HPS) and low-pressure sodium (LPS) appeared on the market … narrower-spectrum sources that emit much more of their output in longer wavelengths like red and yellow”
Question and answer: UPSC's official GS Paper I (2021, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 1 Oct 2026 (how we verify). ·