why does a light bulb need a vacuum
Incandescent light bulb Early lamps, and some small modern lamps used only a vacuum to protect the filament from oxygen. This however increases evaporation of the filament, albeit it eliminates the heat losses.
3-way lamp The two filaments can be activated separately or together, giving three different amounts of light. A typical 3-way incandescent bulb is a 50 W / 100 W / 150 W bulb. It has a low-power 50 W filament and a medium-power 100 W filament. When they are both energized at the same time, 150 W of power is delivered, and a high level of light is produced. Usually screw-base 3-way bulbs fit into regular Type A sockets. Larger 3-way bulbs (up to 300 W) have a larger "mogul" base. These 3 way bulbs can also come in spiral designs. [3]