UPSC Prelims 2021 · Question 72 of 97

UPSC Prelims 2021 question on Sodium vs LED Lamps

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.

  1. 3 only
  2. 2 only
  3. 1 and 3 only
  4. 1, 2 and 3
Show answer

Answer: C. 1 and 3 only

Verdict

Correct statements: 1 and 3 only → Option (c).

Statement by statement

Statement 1 – CORRECT: Sodium vapor lights are omnidirectional. Omnidirectional lights produce light in 360 degrees. This is a large system inefficiency because at least half of the light needs to be reflected and redirected to the desired area being illuminated. Whereas, LEDs are extremely energy efficient relative to every other commercially available lighting technology. They emit light directionally (over 180 degrees versus 360 degrees which means there are far fewer losses from the need to redirect or reflect light). Hence, statement 1 is correct.

Statement 2 – INCORRECT: In some areas (e.g. lifespan) sodium lamps are drastically inferior to LEDs. The extremely low maintenance and replacement costs with LEDs is actually a major cost benefit over the long term. LED lifespan can be greater than 100,000 hours, i.e. more than 4 times than sodium lamps. Hence, statement 2 is not correct.

Statement 3 – CORRECT: Sodium-based lamps produce visible light directly but also in a very narrow frequency range (yellow-orange) so that objects of other colors appear as shades of grey. Such light is referred to as monochromatic. LED lamps offer significant colour advantages.

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