Incandescent bulbs contain filament in them.They use up a lot of energy to heat up the filament and glow.They not only produce light but also produce heat. On the other hand, CFL bulbs contain mercury in them(very little).When a voltage is applied across the electrodes, the gas inside the tube gets ionized, conducts electricity and in the process generates ultraviolet (UV) light. When the UV rays hit phospor coatinf inside the bulb, its lights up. Hence, it uses less energy.
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