Question

A typical 100-watt light bulb consumes 100J of energy per second. If a light bulb converts all of this energy to 500nm light, how many photons are produced each second?

Answer 1

Let us estimate that the average wavelength of the visible light is about 500 nm. This means that the average energy per photon is about

E = hc/λ = (6.626*10^-34 Js)(3*10⁸ m/s)/(500*10^-9 m) = 3.97 x 10^-19 J

to find the number of photons per second falling on these pages, we have to divide 100Js^-1 by the energy per photon

                                            = (100 J/s)/(3.97 x 10^-19 J) = 3.97 X 10¹⁷ /s.