The two types of bulbs (fluorescent and incandescent) use very different methods to generate light. Inside an incandescent light bulb is a thin metal filament. This filament offers a large resistance to an applied voltage causing the filament to get very hot. Because the filament reaches a very high temperature, it starts to glow and emit light. It is not difficult to see why this is a rather inefficient way to generate light.
In contrast, inside a fluorescent bulb is a tube filled with a gas containing vapors of mercury and argon. On either side of the tube are electrodes. By applying a voltage across the electrodes, you force current to travel from one electrode to the other. This current is just a stream of electrons, and when an electron collides with a gas molecule, it causes the gas molecule to emit a photon (particle of light) in the ultraviolet regime. On the inside of the glass tube, there is a phosphor coating. This coating is special because when ultraviolet photons hit the coating and cause a visible light photon to be emitted.
This process is far more efficient than that used for incandescent bulbs, and that is the reason why less heat is generated for fluorescent bulbs.
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