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After long periods of use, a grey spot develops on the inside of a bulb. This is because
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- the tungsten filament evaporates and collects there
- the heat of the bulb scorches the glass at the top
- dust inside the bulb condenses on the top
- glass undergoes a change due to the heat
- the tungsten filament evaporates and collects there
Correct Option: C
At high operating temperatures, tungsten evaporates from the filament and gradually forms a gray film on the inside of the bulb. The inert gas atoms collide with the evaporating tungsten, causing some of the tungsten atoms to return to the filament. Eventually, however, a thin spot develops in the filament. When this happens, the filament will break and the bulb fails.
