Correct Option
The correct option is Tungsten.
Explanation
The operation of an incandescent electric bulb is based on the heating effect of electric current (Joule heating). When electric current flows through a thin filament, the resistance causes it to heat up to extremely high temperatures, resulting in the emission of light. The material selected for the filament must withstand these conditions without melting or disintegrating.
Detailed Analysis
- Tungsten Tungsten: Correct. Tungsten is the preferred material for bulb filaments primarily due to its extremely high melting point (approximately 3,380°C). It also possesses high resistivity and high tensile strength. These properties allow the filament to reach white-hot temperatures required to emit visible light without melting or sagging.
- Copper Copper: Incorrect. Copper is a low-resistivity metal with a relatively low melting point (1,085°C). It is an excellent conductor used for electrical wiring but is unsuitable for filaments as it would melt long before reaching the temperature required to emit light.
- Aluminium Aluminium: Incorrect. Aluminium has a low melting point (660°C) and low resistivity. It is used for power transmission lines but cannot sustain the heat generated in an incandescent bulb.
- Iron Iron: Incorrect. Iron has a melting point of 1,538°C, which is insufficient for incandescence. Furthermore, iron oxidizes rapidly when heated, leading to quick degradation of the filament.
Key Takeaway
Tungsten is exclusively used in incandescent bulbs because of its exceptionally high melting point and ability to maintain structural integrity at the high temperatures required for light emission.