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. ​

The correct answer is C, since statements 1 and 3 are true and statement 2 is false.

• Statement 1 is true. A sodium lamp is an omnidirectional source that radiates light in all directions, around 360 degrees. An LED is inherently directional and needs diffusers or optics to spread its light, so this is a real design difference.
• Statement 2 is false. LED street lights last roughly 50,000 hours or more, while high-pressure sodium lamps last around 20,000 to 24,000 hours. LEDs have the longer life, which is one reason they have replaced sodium lamps.
• Statement 3 is true. Low-pressure sodium lamps emit an almost monochromatic yellow-orange light, which gives very poor colour rendering. LEDs can produce white light with good colour rendering, which helps visibility and night-time safety.

The core concept is the comparison of light sources on efficiency, lifespan, spectrum and light distribution. This links to the energy-efficiency and smart-city street-lighting schemes under urban governance and to the Mains theme of sustainable energy use. Note that LEDs are now the default choice for most municipal lighting, so sodium lamps are being phased out.

Trap: Candidates often assume that the lamp with the higher efficiency also lasts longer, or wrongly credit LEDs with omnidirectional light. Check each statement against the specific property.