A modern 200 W sodium street lamp emits yellow light of wavelength 0.6 `mum`. Assuming it to be 25% efficient in converting electrical energy to light, the number of photons of yellow light it emits per second is

Options

A. `1.5 xx10^(20)`
B. `6 xx 10^(18)`
C. `62 xx 10^(20)`
D. `3 xx 10^(19)`

Answer

A

Explanation

Energy of a photon , `E = (hc)/lamda`
`((6.6 xx10^(-34)Js)(3 xx10^(8))ms^(-1))/(0.6 xx10^(-6))= 33xx10^(-20)J`
Number of photons emitted per seconds is
`N=((25)/100P)/E=(25/100xx200W)/(33xx10^(-20)J)=1.5 xx10^(20)`