A bulb lamp emits light of mean wavelength of 4500Å. The lamp is rated at 150W and 8% of the energy appears as emitted light. How many photons are emitted by the lamp per second?

The energy of photon =  $\frac{hc}{\lambda }=\frac{(6.63\times {10}^{-34})(3\times {10}^8)}{4500\times {10}^{-10}}$
Useful power =  $150\times \frac{8}{100}=12W=12J/s$
Number of photons emitted per second by lamp =  $\frac{12\times 4500\times {10}^{-10}}{(6.63\times {10}^{-34})(3\times {10}^8)}=27.17\times {10}^{18}$