CORDELIA MOLLOY / SCIENCE PHOTO LIBRARY CORDELIA MOLLOY / SCIENCE PHOTO LIBRARY
Light bulbs. The incandescent bulb at left contains a tungsten filament inside the glass, which glows as an electric current passes through it. The spiral compact fluorescent bulb (right) is made of phosphor-coated glass tubes filled with inert gas and mercury vapour. When electricity passes through these tubes it excites the mercury vapour 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.
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