Question

THE BRIGHTNESS OF the BULB IS OBSERVED TO DECREASEs gradually with its period of use beacause of

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 $R=\rho\frac{l}{A}$, 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.

I hope you understand why brightness of bulb is observed to decrease gradually with its period of use.

Answer 2

There are two main reason for why an old light bulb emit low brightness than the new one.

They are Electrical Factor and Optical Factor.

The electrical factor.

The metal tungsten which we are using in the light bulb will keep evaporating to the temperate. Even though Tungsten has an ability to survive in high heat, they are metals after all. Every metal melts when get contacted with heat. In this case, Tungsten evaporates day by day. If we increase the voltage to the tungsten, it will evaporate soon. If we kept increasing the voltage, then there will be no more tungsten left to burn. It will poofff out.

The optical factor

At the high temperature, some atoms of the Tungsten filament will leave from its surface in form of water molecule. Do you remember what I said before? “Tungsten will evaporate”. The water molecules that leave the surface will get settled on the inside surface of the glass. This evaporated water molecules will reduce the radius of the light’s brightness over time.