How Everything Works
How Everything Works How Everything Works

Question 919

Why is an incandescent light bulb hotter than a fluorescent light? — TJ, Woodbridge, VA
An incandescent light bulb produces light by heating a small filament of tungsten to about 2500° C. At that temperature, the thermal radiation that the filament emits includes a substantial amount of visible light. But the filament also emits a great deal of infrared light (heat light) and it also transfers heat via conduction and convection to the glass bulb around it. When you put your hand near the bulb, you feel both the infrared light and the heat that has worked its way to the surface of the bulb. The bulb feels hot.

In contrast, a fluorescent lamp tries to produce light without heat. It collides electrons with mercury atoms to produce an atomic emission of ultraviolet light. This ultraviolet light is then converted to visible light by the layer of white phosphor powders on the inside of the lamp's glass envelope. In principle, this whole activity can be performed without creating any thermal energy. However, many unavoidable imperfections cause the lamp to convert some of the electric energy it consumes into thermal energy. Nonetheless, the lamp only becomes warm rather than hot.


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