Light bulbs. The incandescent bulb (left) contains a tungsten filament inside the glass bulb, which glows when an electric current passes through it. Only 10% of the energy produced is light, the rest is heat. The compact fluorescent bulb (right) is composed of phosphor-coated glass tubes filled with inert gas and mercury vapour. The ballast (base) sends electricity through the tubes, exciting the mercury, which causes the phosphor to emit light. Fluorescent bulbs produce less heat than filament bulbs, so more of the electrical energy is converted to light, making them more efficient.