Indiscriminate disposal of used fluore- scent electric lamps causes mercury pollution in the environment. Why is mercury used in the manufacture of these lamps?
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
Correct Answer: Option B
Explanation
1. The question asks why mercury is used in the manufacture of fluorescent electric lamps, given that its disposal causes mercury pollution.
2. Fluorescent lamps work by passing an electric current through a gas (usually argon) containing a small amount of mercury vapor.
3. Statement (B): When the lamp is switched on, the electric current excites the mercury atoms, causing them to emit shortwave ultra-violet (UV) radiations. This statement correctly describes the primary role of mercury vapor.
4. The inner surface of the lamp tube is coated with a fluorescent powder (phosphor).
5. Statement (C) is incorrect; it is the phosphor coating, not the mercury itself, which absorbs the UV radiation and converts the ultra-violet energy into visible light.
6. Statement (A) is incorrect; the phosphor coating determines the color temperature ('whiteness') of the light, not a mercury coating.
7. Therefore, the essential function of mercury is to produce UV radiation when electricity passes through its vapor.