Question

Why do incandescent lightbulbs grow dim late in their lives, particularly just before their filaments break?

Short answer

The cross-section area of the filament reduces as the thickness of the filament lowers, owing to repeated heating and cooling. As a result, the filament's resistance rises, and the current in the filament falls. The voltage across the bulb remains constant, but the current decreases, reducing the power provided to the filament and making the light bulb dim.

Step-by-step solution

Definition of filament break

A fused bulb is one that has a broken filament and does not light up. The circuit is incomplete if the filament of a bulb is broken.

Explaining why incandescent light bulbs grow dim late in their lives, particularly just before their filaments break

Due to repeated heating and cooling and a drop in filament thickness, the filament resistance increases as the bulb ages. As a result, with the same applied voltage, the current flow through the filament drops, and the bulb burns less brightly as it ages.