A bulb emits light of $\lambda =4500\text{Å . }$The bulb is rated as 150 watt and, 8 percent of the energy is emitted as light. How many photons are emitted by the bulb per second?

Energy of one photon $=\frac{hc}{\lambda }$

                                  $\begin{array}{c}=\frac{6.625\times {10}^{-34}\times 3\times {10}^8\mathrm{J}}{4500\times {10}^{-10}}\\ =4.42\times {10}^{-19}\mathrm{J}\end{array}$

Energy emitted by the bulb $=150\times \frac{8}{100}\mathrm{J}$

           $n\times 4.42\times {10}^{-19}=150\times \frac{8}{100}\mathrm{J}$