UPSC Prelims 2010 Q108: Science & Technology – Physics
- (a) A mercury coating on the inside of the lamp makes the light bright white
- (b) When the lamp is switched on, the mercury in the lamp causes the emission of ultra - violet radiations
- (c) When the lamp is switched on, it is the mercury which converts the ultra - violet energy into visible light
- (d) None of the statement given above is correct about the use of mercury in the manufacture of fluorescent lamps
Answer: (b) When the lamp is switched on, the mercury in the lamp causes the emission of ultra - violet radiations
Explanation
Why mercury is used in fluorescent lamps
Fluorescent lamps rely on mercury's specific role in a two-step light-generation process.
| Option | Verdict | Reason |
|---|---|---|
| (a) Mercury coating makes light bright white | Incorrect | This misdescribes mercury's role; it's not a surface coating providing whiteness |
| (b) Mercury vapour causes emission of UV radiation when switched on | Correct | When the lamp is switched on, an electric discharge excites mercury vapour, causing it to emit ultraviolet (UV) radiation |
| (c) Mercury converts UV energy into visible light | Incorrect | This is actually the role of the phosphor coating on the inside of the tube, not the mercury itself |
| (d) None of the above is correct | Incorrect | Option (b) is correct, so this is wrong |
Fluorescent lamp mechanism:
| Step | What happens |
|---|---|
| 1. Electric discharge | Excites mercury vapour |
| 2. Mercury vapour | Emits UV radiation |
| 3. Phosphor coating (not mercury) | Converts UV into visible light |
Answer: (b) When the lamp is switched on, the mercury in the lamp causes the emission of ultra-violet radiations.
Why each option is right or wrong
- (a) Incorrect. Mercury is not a surface coating responsible for 'bright white' light directly.
- (b) CORRECT. The mercury vapour, when excited by the electric discharge, emits UV radiation.
- (c) Incorrect. Converting UV into visible light is the role of the phosphor coating, not the mercury itself.
- (d) Incorrect. Option (b) is the correct description, so 'none of the above' is wrong.
Key point
Mercury vapour, excited by the electric discharge in a fluorescent lamp, emits UV radiation — it is the separate PHOSPHOR COATING, not mercury, that converts this UV into visible light.
Remember
Mercury in fluorescent lamps: excited by electric discharge, emits UV radiation — the phosphor coating (not mercury) then converts this UV into visible light.
Common mistakes
- Confusing mercury's role (producing UV radiation) with the phosphor coating's role (converting UV to visible light).
- Treating mercury as a simple surface coating for brightness, rather than its actual vapour-excitation role.
- Not understanding the two-step mechanism (mercury → UV, phosphor → visible light) behind fluorescent lamps.
Exam tip
Fluorescent lamp = mercury vapour makes UV, phosphor coating converts UV to visible light. Two distinct steps, two distinct materials.
Sources
- Bureau of Indian Standards — IS 2418: Tubular fluorescent lamps for general lighting service
- Hazardous Wastes (Management, Handling and Transboundary Movement) Rules, 2008