After long periods of use a grey spot develops on the inside of a bulb. This is because

After long periods of use a grey spot develops on the inside of a bulb. This is because
1). the tungsten filament evaporates and collects there
2). the heat of the bulb scorches the glass at the top
3). dust inside the bulb condenses on the top
4). glass undergoes a change due to the heat

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