The gas commonly used in light bulbs is argon. Argon is an inert noble gas that does not react with the materials inside the bulb, making it an ideal filling gas for incandescent light bulbs. It helps to prevent the tungsten filament from oxidizing and burning out quickly when heated. In compact fluorescent and LED bulbs, other gases or materials may be used for different purposes, but argon remains prevalent in traditional incandescent bulbs. Its non-reactive nature helps extend the lifespan and efficiency of the light source.