What happens to an incandescent lamp if it is operated at a voltage below its rated voltage?

When an incandescent lamp is operated at a voltage below its rated voltage, it tends to have a longer life. This is primarily because lower voltage reduces the amount of current passing through the filament, which in turn lowers the temperature of the filament. Incandescent bulbs operate by heating a tungsten filament until it glows and produces light; when the filament is heated to a lower temperature, it experiences less thermal stress and is less likely to suffer from the wear and tear that lead to burnout.

While the reduced voltage might impact the brightness output of the lamp, the key factor in extending the life of the bulb is the decreased thermal stress on the filament. Therefore, though the lamp may not emit as much light, its operational longevity benefits from the lower voltage conditions.