I was staring at a burnt out light bulb today and suddenly wondered what is the physical difference between bulbs of different wattages? Internet to the rescue. The longer the filament, the brighter the light. In a typical 60 watt incandescent bulb 22.8 inches of tungsten wire filament is formed into a coiled coil (Wikipedia cites General Electric Technical Publication TP 110 “Incandescent Lamps” as the source of that measurement). The bulbs that have 3 brightness settings have two filaments and the switching turns on one filament or both.