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A 150 W lamp emits light of mean wavelen...

A 150 W lamp emits light of mean wavelength of `5500Å`. If the efficiency is `12%` , find out the number of photons emitted by the lamp in one second.

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`{:("Number of photons emitted per second n" = (P lambda)/(hc),,P = 150 W),(,,lambda 5500Å),("If the efficiency is 12%," eta = (12)/(100) = 0.12,,h = 6.6 xx 10^(-34)Js),(n = (P eta lambda)/(hc),,c = 3 xx 10^(8) ms^(-1)):}`
`= (150 xx 0.12 xx 5500 xx 10^(-10))/(6.6 xx 10^(34) xx 3 xx 10^(8)) = (99000 xx 10^(-10))/(19.8 xx 10^(-26)) = 5000 xx 10^(16)`
`n = 5 xx 10^(19)`
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