According to Union College, an incandescent light bulb operates by acting as the load in an electric circuit. The glass bulb contains a wire which heats up with electrical current, and the heat causes the wire to glow white-hot and emit visible light.
The small wire inside a light bulb is called a filament, and it's made of tungsten. Tungsten is used because it has an extremely high melting point of 6,580 degrees Fahrenheit and because, as a metal, it can conduct electricity. Oxygen is removed from the bulb and replaced with another gas that doesn't allow combustion in the extreme heat; therefore, the tungsten glows white hot. The light bulb has the tungsten filament attached to a metal base that makes contact with the interior of a light socket, allowing an electrical circuit to form as long as the light switch is in the on position. If a small gap forms in the glass of a bulb, air immediately displaces the gas, causing the tungsten to ignite and leaving black residue on the interior of the bulb.
Incandescent bulbs release the majority of their energy as heat, with the light existing as a side effect. The heat is what causes the glass to weaken over time as the molecules expand and contract.