The gas commonly used in light bulbs is argon, often combined with nitrogen. In incandescent light bulbs, argon serves as an inert gas that prevents the tungsten filament from oxidizing and burning out quickly, thereby extending the bulb's lifespan. For fluorescent and CFL bulbs, they contain a small amount of mercury vapor along with gases like argon to produce light. These gases help create a conductive environment for the electric current to flow, which ultimately produces visible light. As a result, argon plays a vital role in enhancing the efficiency and durability of various types of light bulbs.