Question

Why is the bulb is evacuated​

Answer 1

Answer:

In order to prevent rapid oxidation of the filament, one places it in a glass bulb, which is either evacuated or (more frequently) filled with some low-pressure inert gas like nitrogen, argon or krypton, which slows down the evaporation of the filament material but also carries away some heat.

Answer 2

In order to prevent rapid oxidation of the filament, one places it in a glass bulb, which is either evacuated or (more frequently) filled with some low-pressure inert gas like nitrogen, argon or krypton, which slows down the evaporation of the filament material but also carries away some heat.