Question

Brightness in bulb decreases gradually with its period of use due to

Answer 1

as you know, light bulb is made of a tungsten wire that has been kept a high temperature for many hours . High temperature evaporate tungsten from filament, decreases its radius . from , we see decreased cross sectional area leads to an increase in the resistance of the filament. this increasing resistance with age means that the filament will carry less current the same applied voltage. With less current in the filament, there is less output and then filament glows dimly.