Question

A special kind of light bulb emits monochromatic light of wavelength 630 nm. Electrical energy supplied to it at the rate of 60W, and the bulb is 93% efficient at converting that energy into light energy. How many photons are emitted by the bulb during its life time of 730h?

Answer 1

find the energy per photon (E=hc/Lambda)

find the total energy: 60*730*3600 watts

numberphotons*.93*hc/lambda=60*730*3600

solve for number photons

check my thinking.

Answer 2

The correct answer should be numberphotons*hc/lambda=.93*60*730*3600 because the 93% efficiency applies to the bulb and not the energy per photons.

Answer 3

Power=energy/time
If power is 60 watt that is 60 joule per second.

Energy emitted for 730 hrs=60*730*60*60=157680000J

93 percent of total emitted energy=0.93*157680000=146642400J

Energy of single photon =hc/lambda=3.155*10 pow(-19)


Then no:of photons emitted by the bulb during its life time of 730hr =146642400/3.155*10pow(-19)
=4.64*10pow(26)photons.
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