Discussion
A tungsten filament lamp has a low efficacy because most of its input appears as
Answer: B
Correct answer: heat and infrared radiation
A filament emits by being hot, and a black body at 2,800 degrees radiates overwhelmingly in the infrared - only two or three per cent of the input emerges as visible light. The efficacy is therefore about 15 lumens per watt against 120 or more for a diode source, and the difference is radiated heat.
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