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Estimate how many visible light photons ...

Estimate how many visible light photons a 100 W light bulb emits per second . Assume the bulb has a typical efficiency of about 3% (that is 97% of the energy goes to heat)

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Let's assume an average wavelength in the middle of the visible spectrum, `lambda`=500 nm. The energy of each photon is E=hf=`hc//lambda`.Only 3% of the 100 W power is emitted as light, or 3 W= 3 J/s. The number of photons emitted per second equals the light output of 3 J per second divided by the energy of each photon.
The energy emitted in one second (= 3 J) is E=Nhf where N is the number of photons emitted per second and `f=c//lambda`. Hence
`N=E/"hf"`
`=(Elambda)/(hc)`
`=((3 J)(500xx10^(-19) m))/((6.63xx10^(-34) J.s)(3.0xx10^8 m//s))=8xx10^18`
per second , or almost `10^19` photons emitted per second , an enormous number.
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