This is related to Planck constant. The energy(E) generate by emitting a photon with wavelength `lambda` and velocity c in one second is given by;
E = `(hc)/lambda` where h is the Planck constant.
c = velocity of light = `3xx10^8m/s`
`h = 6.6xx10^(-34)`
`E = (6.6xx10^(-34)xx3xx10^8)/(120xx10^(-9))`
`E = 1.65xx10^(-18)J`
Energy consumed by bulb in one hour `= 20xx3600 J`
Number of photons emittedper hour
So `4.4xx10^22` photons will be emitted in one hour.