Discussion
A 60 W light bulb usually produces 860 lumens. What is the luminous efficiency?
Answer: A
Luminous efficiency is the light output in lumens divided by the electrical power in watts, that is, the lumens given per watt.
\[ \eta = \frac{860\ \mathrm{lumen}}{60\ \mathrm{W}} \]
\[ \eta = 14.3\ \mathrm{lumen/watt} \]
So the luminous efficiency is 14.3 lumen per watt, option (A). The options in watt per lumen give the inverse quantity, which is not the luminous efficiency, and 6.97 is not the ratio of 860 to 60. Higher luminous efficiency means more light for each watt, and a filament lamp of 14.3 lumen per watt is in the usual range for a 60 W bulb, far below that of a fluorescent lamp.
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