The correct option is Nitrogen and argon.
Explanation
Incandescent electric bulbs operate based on the heating effect of electric current. A tungsten filament is heated to extremely high temperatures (over 2500°C) until it glows and emits light. The primary technical challenge in bulb design is preventing the physical and chemical degradation of the filament at these temperatures.
Detailed Analysis
To prolong the life of the filament, the environment inside the glass bulb must satisfy two conditions:
- Prevention of Oxidation: If oxygen were present, the hot tungsten filament would react immediately (burn out). Therefore, air is removed from the bulb.
- Retardation of Evaporation: In a complete vacuum, tungsten atoms evaporate (sublime) rapidly at high temperatures, leading to the thinning of the filament and blackening of the glass. To suppress this evaporation, the bulb is filled with chemically inert gases.
Why Nitrogen and Argon?
- Argon: It is an inert noble gas that does not react with the hot filament. It helps maintain pressure inside the bulb, which reduces the rate of tungsten evaporation.
- Nitrogen: Often mixed with argon, nitrogen is also unreactive at these temperatures and helps prevent electrical arcing (sparking) between the lead-in wires.
Other options such as Oxygen (supports combustion) or Hydrogen would lead to chemical reactions or safety hazards, making them unsuitable for this purpose.
Key Takeaway
Inert gases like Argon and Nitrogen are used in electric bulbs to prevent the oxidation of the tungsten filament and suppress its evaporation, thereby extending the bulb's operational life.