Monochromatic light of frequency `6.0 xx 10^(14) Hz` is produced by a laser. The power emitted is `2 xx 10^(-3)` w. The number of photons emitted, on the average, by the sources per second is

Explanation

(a) Each photon has an energy
`E = h ν = ( 6.63 xx10^(–34) J s) (6.0 xx10^(14) Hz) = 3.98 xx 10^(–19) J `
(b) If N is the number of photons emitted by the source per second, the power P transmitted in the beam equals N times the energy per photon E, so that P = N E. Then
`N=P/E=(2.0xx10^(-3)W)/(3.98xx10^(-19)J)`
`= 5.0 xx10^(15)` photons per second.