Question

A lightbulb has an effciency of 8.2%. How much light energy (not heat energy) is generated by the bulb every second if the buld has pwer of 21w?

Answer 1

It really depends on what the bulb is being used for, since efficiency means how much of the output is USEFUL.
If the bulb is being used for light in a dark room, then it produces (8.2% x 21W) = 1.72 joules per second of light energy.
If the bulb is being used to keep a hamster cage or a fish tank warm, then the 8.2% is the useful part, and the light is the other (91.8% x 21W) = 19.28 joules per second.