Why halogen lamps have long lifetimes?
As the tungsten filament inside a lightbulb is heated, minute particles of tungsten evaporate. Over time, these particles are deposited on the inner surface of the bulb, causing the bulb to blacken. As it loses its tungsten, the filament eventually breaks and the bulb “burns out.” A remedy is to replace the air inside the bulb with a halogen gas, such as iodine or bromine. In such a halogen bulb, the evaporated tungsten combines with the halogen rather than depositing on the bulb, which remains clear. Furthermore, the tungsten becomes unstable and splits from the halogen when it touches the hot filament. The halogen returns as a gas while the tungsten is deposited onto the filament, thereby restoring the filament. This is why halogen lamps have such long lifetimes.
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