Light bulbs. The incandescent bulb (right) contains a tungsten filament inside the glass, which glows as an electric current passes through it. The compact fluorescent bulb (left) is made of phosphor-coated glass filled with inert gas and mercury vapour. When electricity passes through the mercury vapour, it becomes excited and causes the phosphor to emit light. Only 10% of the energy in filament bulbs is given off as light, the rest is heat energy. Compact fluorescent bulbs produce less heat than filament bulbs, so more of the electrical energy is converted to light, making them more efficient.