Electric light bulbs glowing with different levels of brightness. The brightness of an electric light bulb is determined by the amount of electrical current flowing through it. Light bulbs and their sockets are designed for optimum voltage levels. Fitting an incorrect bulb can lead to a dim light or cause the bulb to burn out too quickly. The light of an electric light bulb is produced by a glowing metal filament surrounded by an inert gas contained in a glass bulb. As electricity passes through the thin metal filament, the filament heats up and gives out heat and light.