There are different kinds of light bulbs. The "oldest" light bulb is called "incandescent light bulb", it is what Thomas Edison is famous for.

In an incandescent light bulb, a filament (could be carbon or Tungsten filament) is heated to very high temperature until it glows. But with such high temperature, the filament can be very easily oxidized. Hence inside the incandescent light bulk, it is usually either vacuum, or filled with inert gas.

The inert gas does not affect the brightness of the light bulb, but it does significantly increase the lifetime of the light bulb because it reduces oxidation.

A slightly newer light bulb is the fluorescent lamp. Inside the fluorescent lamp there is vaporized mercury, which is still a gas (but very toxic). Mercury is very good metal, and its electron can be excited and then emit UV radiation. The emitted UV radiation will turn into visible light when it hits the fluorescent coating of the lamp, this is why the fluorescent lamp is not transparent: it needs a coating on the light bulb/tube.