Monochromatic light of frequency `6.0xx10^(14) Hz` is produced by a laser. The power emitted is `2.0 xx10^(-3) W. `(a) What is the energy of a photon in the light beam? (b) How many photons per second, on an average, are emitted by the source?

Explanation

(a) Each photon has an energy
`E=hv=(6.63xx10^(-34)Js)(6.0xx10^(14Hz))`
`=3.98xx10^(-19)J`
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)`
`=5.0xx10^(15)" photons per second."`