A light bulb is nothing more than a glass vial containing a tungsten wire and an inert gas. During its normal life, tungsten evaporates due to incandescence; the more efficient the bulb, that is, the hotter it is, the more metal evaporates. The evaporated tungsten ends up condensing on the inside of the ampoule . If it had been done under vacuum, the blackening would be uniform throughout the bulb, but the existence of the gas causes thermal currents to appear and the vapor is deposited on top.
Why do light bulbs explode when heated in the microwave?
Why do halogen bulbs burn out when touched?
What is the difference between white and yellow bulbs?
